So almost everyone, including me, is going to tell you that if you want to do ortho, then research in ortho is ideal. By research, I mean actually publishing or at least poster presentation at a conference, not just being 4th author. Having said that, I did research in pathology, and I luckily got something published in a year. I decided to not do path, and every specialty that I have been interested in (med peds, gen surg, and now ortho) have told me they don't care what I did research in, they just care that it was research that I got published. The chairman of ortho at my school, who has also been residency director and other positions at 2 other academic programs actually told me not to waste my time doing ortho research since I already had a paper published. It showed that I have done research, understand it, and can do it again. If you really like rads, I would do research in rads, but don't worry too much if you switch later, it won't all be a waste. However, if there's a high likelihood of you doing ortho, I'd opt for ortho. So thats the long answer to your short question.
sscooterguy