Obviously impact drivers and impact wrenches are designed for different purposes, but since they both work on the same principle is there some overlap? Is a small cordless impact wrench just a slightly scaled-up version of a cordless impact driver, with a 1/4" square drive instead of a 1/4" hex drive, or are there further differences? Could I buy adaptors to use one occasionally in place of the other, and if so what would be the disadvantages? I can guess a few downsides: the wrench would be heavy and large when used as a driver so not convenient at arms length or in tight spaces; the driver would be underpowered for really heavy-duty wrenching; the wrench might need some care to avoid twisting the heads right off your screws. Is there more to it than that?
Most driver bit sets come with a 1/4" square drive adapter (my small Milwaukee set also came with a 3/8" drive as well, though I don't know if that's common). With that one bit, your driver is instantly almost exactly the same as a 1/4" impact wrench. You'd have to look at torque specs to see if the models you're considering are comparable, but I bet they are. Those little impact drivers have come a long way -- battery life is great and the torque is hard to believe.
I'd only consider a square drive impact wrench for heavier automotive (or other mechanical) use, and only in a larger size like 1/2" so it can provide some real help in removing tough fasteners.
Impact drivers and impact wrenches are quite different. Look at the torque ratings, and you'll see why I say that.
18V impact drivers: 1300-1400 in-lbs of torque 18V impact wrenches: 3800-5800 in-lbs of torque
I'm just guessing here, but I would say that wrenches have this extra torque because they have lower RPM and IPM (impacts per minute.) By having more time for each impact, they can save up more inertia, delivering about 3.5 times as much torque.
Some confusion arising from the term impact driver being used for two different tools.
There are manual impact drivers and there are electric impact drivers. The manual impact drivers are more specialized tools and have a cammed bit holder with a heavy cylinder that's meant to be struck with a hammer, and used for removal of philips screws. The cam action translates the impact motion into a combination of rotary and downward axial motion, to minimize the chance of the bit camming out of the screw head and stripping it.
Electric impact drivers are the ones most people are familiar with, and accept small 1/4" inserts intended for driving smaller nut and bolt fasteners.
The impact drivers deliver a downward force as well as rotational , so better suited for screw heads like phillips to reduce/ eliminate cam out.