Ashe and Rassler is a might-have-been; he dies before they can find out how just what kind of relationship their marriage could have evolved into. Balthier and Fran, on the other hand, is a what-is, an established relationship, and I've written before on why I think it's such a great relationship, so I won't repeat myself here.
I'm more into Rassler/Ashe being a romantic couple and I flip flop on Balthier/Fran all the time. But I agree with Lic, romantic or not they just have an awesome chemistry that's hard to match.
I'm biased and I like Ashe and so voted for her. And Fran and Balthier just feel like really good work mates and friends. Partners in crime rather than partners.