Pirates>Knights
Basch is great for getting through a tough area where there's a bunch of mobs attacking together. But he's not at all versatile.
Balthier, however, is the go-to guy for tough boss fights. Vaan is my hardhitter/tank so Basch is just redundant. Balthier's guns/xbows have the added status effects that make him much more valuable.
I do keep both of them leveled though.
Outside of battle, there's no comparison. Balthier ^_^