From my experience on my aways, I would say that the trauma rotations have been more fruitful. First, I was more useful on rotations. Second, I got to do WAY more in the OR on trauma cases. Third, the residents appreciated the little things more (like getting the splint material, starting the notes, and taking off casts/splints in clinic). Forth, attending actually knew who I was.
I was able to get great letters from both sub-specialty rotations and from trauma. But what I think mattered more in the long run was bonding with the residents on trauma. I know a few went to bat for me and I think this was worth more in the long run.