If I was out of love I'd approach the cute, casual girl. I've always preferred tomboys, girls with a bit of character. Though if I did that I'd subconsciously be looking for a relationship.
My best friend and I were talking about this engineering girl at uni who's in the year below us. She's really pretty in a girl-next-door English kind of a way, but she's basically a female nerd (no specs though). She dresses in combat trousers and checkered shirts. But whereas I find it intriguing that she's like that, my friend would prefer her to be more dolled up.
I'm not immune, though, to the charms of the 'little black dress' girl in a club. But I'm always really conscious of other people in the club thinking I'm a lech for approaching her. So if I do approach her, I make it less seedy by self-consciously making jokes or just grinning like an idiot.