This is a very tricky question. In an attempt to answer this question, I'd say the importance of both legs and arms to human survival cannot be over emphasized.

First you need to legs to move the body around to different places. And second you need the hands to do jobs, bath and also feed yourself.

Personally I'd say I wouldn't prefer any part of my body to be chopped of for any reason because all parts are equally important and are needed for my survival.

As odd as it sounds, I cherish both my arms and legs the same way because they both help me in getting things done and losing one is equivalent to losing all.

But considering the level of technological advancement now I'm the world, I can easily get a robotic or prosthetic leg so if I am boxed up in a corner to choose between my life or choosing my leg or hand off I'd gladly go for the leg because I know I can manage to balance the leg with an artificial one but not the arm because I'd love to feel my arm around my body always.