Bikini's, obviously. The more skin, the better. I'm sure some one pieces can look great, but bikinis just always look great. Dependent on the chick's body, of course. But, that's the same thing for one pieces. Bikinis are just overall better, like 99% of the time.
I prefer the bathing suite complements her figure. Many one pieces don't do a good job of it. So I like a nice modest one piece that does a good job of complimenting her figure as compared to a bikini.
Bikini. MUCH more revealing. I was at Blizzard Beach at Disney World last summer and in the line for one of the slides I was behind a very hot girl in a small bikini. At certain times I could see a lot more than she'd have liked me to see. Don't get that with a one-piece.
I genuinely think therethere are nice one piece and nice two piece swim suits, but i feel like one piece swimsuits often look more elegant and ladylike, so i have to say i have a slight preference for the one pieces