Depends on what the guy looks like. Like if he has some kind of young look or baby face then clean shaven, if the guy is like a rugged type guy some stubble or short beard. But I don't think it all too much matters.
I definitely prefer decent stubble. It looks sexy and when you just keep it like that, it's soft enough not to irritate my skin. When my boyfriend shaves his face clean, it irritates the hell out of my skin when it starts growing back.
Depends on the guys face... I think stubble is hot, but clean shaven is wonderful too :) just.. no full beards. But the guys at my age (17) they usually can't grow full beards, so its not an issue, except the nasty peach fuzz... *shudders* lol XD