Well, really it depends on whether we're talking about Basch gameplay wise or storyline wise. If it's gameplay, then Basch would win because he has can have access to all the abilities and equipment in the game, as well as all the Espers (should one choose to do so). Storyline wise, Rosa would win because she can attack from a range and heal herself.

Still, I hate FFXII, so I would have to give my vote to Rosa.