Anyone correct me if i'm wrong but I think that three piece cranks should technically be stronger than one piece, as they are basically three sections of straight metal, which should cope with stress better than the one piece, which bend to form the spindle section of the crank.
Saying that, I really don't buy stuff these days based on how it performs but what what was considered cool and what I always wanted but couldn't afford bitd and that for me is three piece any day.