i completely agree with 'smoke this'--i've never heard of a deferment longer than a year. and the thing is, not all schools will even offer this. some explicitly state that they won't allow deferments at all. nothing personal, but i don't think you'll get any special treatment as a rhodes scholar, because your reason for deferring is not necessarily any more 'important' than anyone else's.
if you apply to both and actually wind up a rhodes scholar, i would take that opportunity, because that's not something you can turn down. then you'll just have to reapply to med school in two years. chances are, if you were competitive enough to be accepted now, you'll only be *more* competitive with a rhodes scholarship under your belt, so i wouldn't worry about getting in the second time around.
good luck with whatever you decide.