Surely, by now you realize this question has no real answer. It depends on the individual girl and the individual guy... their chemistry, experience, interest, etc.
... but he'd need to try about 1000 of each to get a representative sample and have some ground to make such a broad-sweeping statement about which is better. But then, why limit it to a fat vs. thin. Why not short vs. tall? Or one hair color vs. another? Or girls from this city or that one?