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I make intensive use of both my hands about 12 to 20 hours every day. Need those in top working order. I will rely on them until death.
If my legs were in excellent health, probably my right leg. Left leg is the "plant" leg for stability in carrying, kicking, etc. Right leg is the precision leg. If at all possible, I'd choose to to keep at least part of the leg, including the knee joint. That gives maximum benefit for prosthetics.
Tougher choice since, in real life, my left knee was pretty much destroyed when I was mugged a few years back. If I could afford proper surgery & rehab to get that knee back into decent working order, I'd keep the leg. But, if that were not an option, my right leg is generally in better overall health right now and I'd probably favor keeping it.
Grim poll. Reminds me of a movie I saw once where a pimp decided to punish one of his whores. He tells her he's going to cut one of her eyes out, and she has to choose which one, right now, or he'll take both.