Hello.
I am currently doing full-time research, after having completed 3rd year of medicine (6 yrs in all).
I have found research to be more relaxing, even though you are expected to put 60 hours of work a week in your project. But once you're at home, you can relax (perhaps read an article in the evening) - no stress about lagging behind in the curriculum - no chapters you HAVE to read for tommorow. It's a flexible job, and needless to say, the more time you put into your research, the better the outcome. Furthermore, I have learnt some basic research techniques which I wouldn't have learnt otherwise. This will be extremely useful for me in the future.
To answer your question concerning forgetting the stuff you have learnt in your preclinical years: You'd be surprised how much a difference a year makes. I thought I would be revising throughout my research year, but that doesn't really happen (might work out for you though). As far as theoretical knowlegde is concerned, I really have forgotten alot! Since I have not yet started my clinical studies, it's hard to say whether it will affect me then.
Good luck with whatever you may do!
...Singh