I agree with JP Hazelton...research is research as long as you can discuss it intelligently at your interviews. If you decide you want to do plastics, you will also need to explain why you did the switch over from urology.
Working with the plastics guys is a good idea if that's where you think you're going...not so much from the research aspect but from the connections/network/letter of recommendation standpoint.
What I really think you need is to figure is this: Urology and plastics, while both competitive, are not the same. As I've said before, you need to ask what you see yourself doing everyday for the next thirty years. JP is also right in that in order to get a handle, you need some experience with what each specialty does. It may not be a bad idea to ask to shadow an attending prior to you third year to get some experience.
--M