If you want a focus system to essentially replace having to carry around vinyl or CDs, without being to worried about midi/ableton integration/effects/multi deck sync/ya de da... then Serato.
It is excellent, due to its simplicity.
If you want to get technical on the bidnez, be able to choose higher end soundcards, send midi clock, mix up to four decks at a time, etc, then Traktor.
I switched from Final Scratch to Serato for stability (in the old days), then from Serato to Virtual Vinyl because I was the Numark Rep, then was going to go back to Serato, but went to Traktor, because it would let me mix with three turntable timecode control (four actually).
No-days, I dont even bother plugging in the turntables, I just feed three 'decks' straight into a mixer, and use the forth as a delay/effects loop..
If none of that made sense, then Serato..