Personally, I'd choose the leg because you can do so much more with your arms. Yeah, so you wouldn't be able to walk without assistance but arms are slightly more useful in comparison.
That's going on what I'd prefer to lose, I've broken both my legs before at different times, got through it fine. Never broken my arm admittedly, but wouldn't want to try it.
If I had to lose a limb or break a bone again, I'd rather it be my leg (my left one to be precise, I kinda like my right one. There's less problems with it, plus it'd be a pain in the ass to get the coordination up to scratch using my left leg on the bass on my drums)
Edit: obviously being able to play games with hands does tip the balance slightly as well...