MHO, arm trouble is preferable to back trouble. With arm trouble, the problem is usually a major ligament, worst case scenario for the most part is you have some surgery and have a puncher's chance to bounce back. With the back, there's only so much a surgeon can do. All those little bones each of which have a set of ligaments, any of which could be the problem -- the problem is also far more likely to be the bone itself. You can "solve" a major problem with surgery but even if you do it can come back to haunt you anytime, seemingly at random. As long as you're pitching professionally, the factors of stress and repetition that caused the damage are likely still going to be there.