Not an easy one at all.
Barbie is something special. Her multiple-name thing is annoying, but you can't take your eyes off those tits. And bit tits aren't her only thing; she has gorgeous hair and a very nice face. (The pictures at Freeones really don't do her justice, IMO.)
Jennifer looks like the sexy cheerleader from an American high school. The girl you lusted after all through your teenage years, the one you'd probably have given your left nut for a chance to fuck. She's got that amazing "mostly innocent" look... you know, the kind of girl that can fool her parents into thinking she's still a virgin, but you know once you get her in the sack she's going to be a minx.
Tough, tough, tough.
But that multiple-name thing is annoying. I'm going for Jennifer.