Depends. If I'm doing this for life (unless I'm being forced at gunpoint) then I'd assume the ugly girl (You know, it feels weird saying that, can I call the one OP called ugly Derek and the hot one Paul?) is really nice and we get on really well etc, which is why we're having great sex. Similarly, if sex with Paul isn't that great it probably means we don't get along and don't know how to please each other or particularly want to learn.
Following from this, since the question was the rest of your life I'm gonna go with Derek, since I'd much rather be having great sex with someone I like than OK sex with someone I don't.