Final scratch 2

Just make sure you buy the full version of traktor 3 and don't settle for the cut down version.
4 decks, looping, FX, much better search and filing and a decent graphical layout with regular software updates.

Serato holds up better for turntablism and scratching, but the interface is nowhere near as nice as traktor 3.

Depends on your style though. They have different strengths. Depends on how you mix, and whats important to you.