It may depend to some extent on what your swing is like for these shots. A lot of people prefer a swing arc that just brushes the grass, with the back side of the flange (the bounce) being the only part of the club to actually contact the ground. For that kind of shot, use a club with more bounce, you'll have a little greater margin for error.
It may also depend on the firmness and speed of the greens you play. The 56 should fly higher, land softer, and stop quicker than the 50, everything else being equal.
Personally, I use my 60/8 lob wedge for most short game shots. I've played and practiced enough to be able to close it down and hit low runners, open it up and hit high soft lobs, and almost everything in between. The only real change is when I get close enough to hit a true chip and run, and I'll use anything down to 7 or 8 iron, but those shots aren't what you're asking about.
So just a thought, have you been hitting off of mats only for some period of time? My left hand, specifically my ring finger below the knuckle gets sore. I put Blue Emu on before I play and it’s fine. We’ve been on mats all year and that possibly could cause some pain. Or perhaps an early symptom of arthritis? I’m 65 so that could be a possibility.
I've been using my 56 degree sand wedge about anything that's too close for full swing (at least until I get to chipping range), but I'm wondering if maybe I should be using my 50 degree gap wedge instead. What do most people use for these types of shots?
I got this problem from playing the corded grips that came with my iron set. Since I didn't have the problem with my old set it was an easy diagnosis.
There's a product called Sting Stoppers. They insert into the shaft of your clubs. They actually work. Also you might try Golf Pride CP2 Pro Grips.