Arm. I've had crutches before and they're not fun. If I had the choice of which arm would be broken, my left would be it. I don't do much but type with it.
Leg... then I could get time off work, with worker's comp, and get a sorry sonu***un to push me around, and/or hit people with crutches when they made me mad, and make everyone sign my cast and simultaneously kiss my foot >
How badly is the arm broken? For as far as I know, you can still do alotta things with your arms. I've once seen a kid with a broken arm playing a vid. game. It may not be as comfortable, but I prefer the arm. Although legs get you around, it's not so useful when it's broken (unless you have crutches, which can be used as a self-defense weapon)
I would rather have my right arm broken because if i had my leg broken i couldnt really walk or play sports.And i would want my right arm broken because i write and do a lot of stuff with my left arm.