There is a review posted on digitaldjtips.com with a comparisson of Traktor Duo and Traktor Pro.
A pretty nice review with alot of good pointers to the fact that Traktor Duo is a pretty sweet deal for those of you who are just using Traktor to mix two tracks together.
I mostly use five effects and two of them are not included in the Duo package. (Beatmasher 2, Gate, Delay, Turntable FX, Iceverb)
The Quantize function is for me a winner. You are always in sync even though the sync button isn't active! This also makes it easier to use four decks.
The master clock is mainly for those of you who want to use external software like Abelton. Or if you want to connect to other Traktor DJs and play together on two or more computers, but still have the option to sync.
The thing for me about Traktor is that it DJing is now so much more then just mixing two tracks. It's about exploring the possibilities of music and fully enjoy what Native Instruments Traktor is all about. I also seldom use deck C & D, but I'm getting there. It's about expanding your realm as a DJ and reaching another level of experiencing and presenting music. So do you realy want to be stuck on two decks and limited effects?
Read the review and see what you think. Personaly I would go for Traktor Pro. The pricepoint isn't that much higher. Atleast not now when the prices are 50% off!