this is a very tricky one. I seriously enjoy the sense of comfort from resting my head on some large tittys, and love the way it makes a girl go wild when you suck on em just right.
I also love the way a girls *** looks (if it's a nice ***), enjoy grabbing it during cowgirl and spreading her cheeks, but most girls are self conscious about their ***** (girls that aren't slags), so most forms off *** play are off limits with your average chick. this makes the *** more of something to be appreciated from a distance, whereas all girls love to have their nipples played with.
for these reasons I prefer tittys, it's much more attainable and I'm pretty sure girls appreciate it more when you give their tittys some attention.