Traktor. Serato's advantages, in my eyes, are plug-and-play setup, consistency between DJ setups (it's easy to jump on someone else's Serato setup and know what you're doing), and vertical waveforms for easier/visual beatmatching.
Traktor, on the other hand, has 4 deck mixing (without shelling out for an SL4/Rane 68), more robust/plentiful effects, sync for when you want it, internal mixing and in-depth midi capabilities, soundcard flexibility (when not using DVS), 8 cue points per deck instead of 5... The list goes on. Oh, and Traktor's substantially cheaper for comparable setups.
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