I prefer guy straddling her, squeezing her tits together (nipples visible, still), and fucking the shit out of her tits until cumming between them and shooting either into her neck and chin, or on her face. On top is kind of boring to me, honestly, though it can be fun from time to time too. I know I personally prefer to be on top when I'm tittyfucking someone, but I've also let her do all the work a few times, and that feels good too, but I find that when I'm on top and squeezing her tits together, I have a little more control over how tight I can press her boobs together, without constantly having to be like, "Oh, squeeze tighter." I honestly just love tittyfucking, so the position doesn't matter that much in the end.