Which tool would be best suited for various around the house jobs? I already own a cheap cordless drill and the battery frequently dies. I need to know which tool is best for hanging light fixtures, TV's, product assembly, and even screwing in fallen fence pickets.
You can't go wrong either way...
The impact driver is generally considered a little better for driving screws, the drill is better for drilling holes. But they are both adequate for maybe 90% of general DIY tasks around the house, so I don't think it's a big deal either way.
I have impact drivers and cordless drills in 12V and 18V, and I will reach for whatever is closest for most tasks. If I am picking one out of the truck, the 12V impact gets the most use - it's smallest and lightest and more than adequate for most tasks.
Consider the chuck
Impact drivers come with a 1/4" hex chuck, so if you have drill bits with a regular shank, you're probably better off with the drill, so you don't have to replace your drill bits. (You can buy drill chuck adapters for hex chucks and 1/4" adapters for drill chucks, so again it isn't a deal breaker, although I'd prefer not to monkey with an adapter.)
Drills are more controllable
The trigger on a variable speed drill is easier to finesse than an impact driver.
Most cordless drills also have a very useful clutch setting, you can adjust the torque and avoid overtightening things, stripping screws, overdriving, breaking cover plates etc. (Some impact drivers have limited torque adjustments, but not as granular as the dial on drills.)
So if you're still on the fence, here's what I'd recommend. If you don't mind buying new drill bits, I'd probably go with the impact driver. They're nice and compact, the hex chuck makes bit changes quick and easy, and they're generally easy to handle.
For small holes and spade bits, the impact is perfectly fine for drilling. Impact drivers drive long screws better than a drill. If you're hanging TVs, the impact driver will drive 1/4" lags into studs very easily. With the impact driver, you may find yourself using screws for a lot of tasks you currently use nails.
I've got both (both Makita LXT tools). They're very different devices, despite the visual similarity, but at a push both can be used in place of each other. I also own a pillar drill and a mains SDS drill.
But if you've only got one, for general DIY, get a cordless drill (and get a good brand one), as it's a lot better as a driver than an impact driver is as a drill.
A drill can drill excellently – not as well as a pillar drill for precision or power (e.g. if you're drilling steel), and not as good as an SDS for concrete / engineering bricks, but pretty well in most circumstances. A good cordless drill should still be ok for smaller holes in steel, and may be ok on brick/concrete depending on your drill, the hole size, and the hardness of your bricks.
It can also drive screws fine in most scenarios, but will struggle with longer screws, e.g. decking. They're usable with sockets, though I don't do enough of that type of work to really say how good it'd be for e.g. car mechanics.
Because the speed is adjustable with trigger pressure, they're great for assembling bolts in flatpack.
An impact driver excels at driving screws, and drives long screws with ease, and much, much faster than a drill does. Get one if you ever board a loft or do decking, or any other task that involves lots of screws.
It'll drill a hole in plasterboard with ease, and it's passable in wood with smaller bits (you'll need the more expensive hex drill bits), e.g. pilot holes. But it's nowhere near as good as a drill compared to using a drill to drive screws. Don't bother using it for brick or steel.
They're also not good for assembling (some types of) flatpack furniture – they tend to over-tighten bolts, and can over-drive screws, which can lead to putting a screw or bolt through the front surface of furniture (flatpack often has very little leeway for over-driving screws), or cracking the surface of furniture (if it's got a plastic or veneer finish).
My purchasing recommendation would be:
A good cordless drill
SDS drill – if you need it for your brickwork. I'm in the UK so houses are brick, but even still, many houses the bricks are drillable with a cordless drill on hammer setting. Unfortunately, mine are super hard.
Impact driver – nice to have, but less useful; get one the first time you have to drive hundreds of screws.
Pillar drill – great for woodwork & metalwork, but not really a DIY tool for 'household' jobs.
Make sure you buy a good brand if you can afford to – personally, I went Makita, based on what friends and contractor I know use, but having had a cheap one, a good brand really does make a difference.
Buy all your cordless tools from the same set, so they share batteries. I now have ~8 Makita LXT tools (including a handheld hoover – great for keeping the house clean!). You'll save a lot of money and hassle having all your tools share batteries.
Hands down if you had to choose one, it's the drill. Impact driver has its use but is loud and clunky in operation which is unnecessary and annoying for normal work.
It is rare that for small projects you will encounter a need for an impact driver. In 10 years I have used mine twice and I have done many DIY projects. Really I probably could have used it more but I guess instead I used my handy vise-grips or some other tool instead since I never got in the groove of using the impact driver despite coming with my set.
Another person said you might need the impact driver for brick or concrete, but since you mentioned small projects, I doubt you're talking about anything which a drill won't do the trick.
Overall my choice is Drill, if you have to choose one. However, for small project you might as well just get one of those starter kits that comes with both. Just my 2c.
You probably want a good example of each one:
Drills are good for cutting holes, buffing, etc. because their smooth motion is entirely predictable...well, at least until you hit a hard spot in the material (knot in wood, etc.) or you're about to break through the other side. If you know that a screw is going to go in easily and then stop, then the torque limit can be useful too.
Impacts are good for driving screws that are stiff all the way in (like wood screws) because the period of zero torque between impacts allows the bit to seat back down in the screw head. This reduces the likelihood of it "camming out" and stripping the head before you even register that something is wrong.
The impacts themselves can also be useful to knock corrosion off that a smooth-motion drill would simply "lean on" and go nowhere against.
In either case, you still have to pay attention. Drills and impacts both can get away from you in a hurry, and their "feel" in your hand is not always accurate. (I've buried a screw head about an inch (2cm) below a wood surface, and it didn't feel any different.) So you still have to watch it and stop at the right time. The ability to "throttle" it is good too - to controllably run slower than max.
One more point that you didn't ask for, but I think is important to mention: Cordless tools of either kind typically have brakes, whereas corded tools generally don't.
The reason for this is that the cordless tools run on DC batteries, and therefore have DC motors, usually the permanent magnet type. This type of motor is automatically a generator as well, and can be shorted across its terminals (battery disconnected) as a brake. Some control schemes do this automatically anyway, but it's not that hard for a manufacturer to add, and it gives a lot more controllability. When you release the trigger, it stops now!
Corded tools run on AC, and therefore have AC motors, usually either the "universal" or induction types. Neither of these are generators automatically, but they can both be made to do that. Unfortunately, it actually takes some doing this time - some serious, intentional engineering - and so it just isn't done for most of the hand-held tool market. So they coast to a stop instead.
If you're good at pulling the tool back at the right time, disengaging quickly from the work while it spools down, or if you're good at predicting how it will spool down with "this load" still engaged, then a corded tool can be made to stop working at a precise time. Otherwise, go for cordless, even if you end up using it with an AC adapter most of the time.
An impact driver is recomended for drilling into harder materials like bricks and concrete masonry units (CMU's, or commonly --but incorrectly-- referred to as cinder blocks). But for softer materials like wood, plaster and drywall a hammer drill is considered overkill; a simple drill will suffice. Overall I would suggest using a tool that covers all the bases, like an 18 volt hammer/drill/driver. One tool gives you three options. And as with any consumer product if you pay a little more up front for a better quality item it will usually be worth it in the long run, rather than going cheap, and having to replace it three times.
What is your house made of?
If you have walls all made of plasterboard (sheetrock), then a cordless drill without hammer action may be all you need. If you have brick walls you need hammer action. If hard brick, stone or concrete, you need an electro-pneumatic drill with SDS+ bits! (Or very sharp TCT bits with an ordinary hammer action, and they will rapidly go blunt).
In my old residence I had a cordless drill with rudimentary hammer action, and it did all I asked of it, including holes for plastic wallplugs (anchors) into soft brick walls. When I moved, it was as much use as a chocolate teapot for trying to attach things to hard brick and stone walls. I went out and bought a mains-powered electro-pneumatic drill (Bosch's cheapest and least powerful) which made life easy again.
The old drill is still adequate as a screwdriver, into softwood or plastic plugs. I'm not sure what I would use an impact driver for. I thought they were for garage mechanics and suchlike, trying to remove rusted steel screws and nuts before resorting to cutting them off?
Around the house/garden:-
A hammer drill is great - get one that plugs into the mains rather than a battery device; make sure you buy some masonry bits.
A modest electric screwdriver (battery powered) is sufficient unless you need to screw in dozens of screws.
A set of hand screwdrivers is a must.
I've never used an "impact driver" - you can always hit a recalcitrant screw with a hammer if you need to give it some impact!