I don't understand why this is an "either or" question. I don't like either. IMO, the hottest chicks are athletic. Neither extreme is particularly appealing.
Honestly a tough call. You can't just look at that one aspect. You have to take a look at the whole picture. I have gone for the super skinny girls in the past and I have gone for girls that I would not consider fat, but are a little bigger than average. It truly all depends on the girl. Could just be because I don't really have a "type".