I use Balthier more often than Basch for 2 reasons.
1.) I find that Balthier's guns, crossbows, and magick are more useful than Basch's raw strength. Besides, Ashe, whom I like better, can be just as effective as Basch in terms of dealing damage.
2.) Balthier is a pirate made of raw, untainted PWNAGE, PIMPAGE, and SEXY, and Basch is alot more boring...And cliché. I mean really...HE WAS FRAMED BY HIS EVIL TWIN BROTHER! Where's the originality in that? Though, Balthier is pretty cliché too.
In conclusion, Balthier is superior to Basch.