I'm in need of installing a shut-off valve for my hose bib (sillcock), specifically a ball valve. But I was wondering if a threaded connection was better than a sweat in terms of effectiveness and maintenance.
In terms of effectiveness there's no difference between threaded or sweat connection. In terms of maintenance, especially for a ball valve, the only maintenance to be done would be replacement upon failure. If that happens regularly threaded connections could be convenient.
Sweat could be preferred when the job is straightforward, permanent, it's inconvenient to measure and thread pipes to length, or the assembly must be built up starting from both ends and meeting at a joint in the middle. In this latter case threaded connections would require use of a union, whereas sweated connection could use a simple coupler - even a "no-stop" coupler if the fit is very close. This sounds like it might be applicable to your situation.
Threaded could be preferred when certain conditions exist. For example: if the assembly is temporary and the valve would be re-used elsewhere later, if fine-tuning the rotation of the valve could be necessary, if there could be a connection to a plastic pipe component near enough that heat from sweating could cause damage, if the person doing the work and the tools available aren't adequate for the sweating job (large pipes, for instance), or if the parts must be assembled wet.
Neither is inherently better nor worse. They are different, but functionally, you get them together not leaking and there they sit. They should not require "maintenance" in and of themselves - if you forget and leave the hose on in freezing weather, you will have to fix them back as far as the damage extends, but if you did that once, you can do that again.
As mentioned in comments, there are also the (expensive) solderless connectors now.
Some folks are more comfortable with a wrench, some with a torch. Soldering pipe was considered a basic homeowner skill once upon a time, much less so these days, thus the rise of the solderless connectors (much less skill required, no flames to be careful of.)
I've had better luck sweating/soldering Cu connections than the threaded-with-tape approach. Another option is to use SharkBite (tm) or equivalent push-on connectors.
FYI, I just added a hose bib out the front of the crawlspace. 8 fittings total with 6 soldered, 1 Sharkbite, and one threaded. The only one I had a leak issue with is the threaded joint.
Edit 1 - Nap should be Mapp
If you're going to go the solder route, I highly recommend a Mapp torch rather than a propane one. The Mapp heats up the copper a lot faster than the propane.
I kind of enjoy soldering when it's not in an awkward position, which is almost never the case for me with plumbing tasks. I'd say I trust soldering most though, so if it's going inside a wall or other place where I really don't want a leak, it's worth the trouble.
For shutoff valves, I don't mind a mechanical connection type fitting. The valve is probably going to fail before the compression joint or the sharkbite connection. They do cost more, but the cost difference is pretty reasonable for the time they save. I didn't trust sharkbites at first, but at this point I'm satisfied they're very reliable when properly installed.
I don't have as much confidence in the copper crimp tools, but since the tools are way out of my price range for the small amount of plumbing I do, it doesn't really matter.