Would you rather have sex with a woman who is four feet tall or eight feet tall?
I have no preferance so far as height is concerned. Both are delightful in their particular ways. This holds true in both 1st and 2nd Lives. In a purely SL context; however, coupling with someone closer to ones own height, around 7' 4" in my case, does tend to help the poseballs line up better. That said, height mismatch is still not really an issue. The visual aspects - animations, camera angles and such - are but a portion of the whole experience. Creative emoting trumps visuals every time. If that emoting makes use of the disparity in interesting ways, all the better.