Leg. I’ve seen people with both and I think it would be easier to do the things I love with a prosthetic leg. I’ve been working with a friend to help design a prosthetic arm device to support his paralysed arm. He’s adapted to riding a bike well, but it’s tough.
This is so difficult. I think it’s not totally fair for me to answer the question because I can’t put myself in the position of someone who has a prosthesis. I certainly don’t want to imply that one is luckier than the other. You’ll just have to vote for one of the other engineers on this one!