Having ridden both and not blindly presuming, MAVIC!!!!!! First, Mavic SLR breaking is far superior to the Zipps, you will never have to worry about them on descents as you might with the Zipps. The Mavics are stronger, come with tyres and wheel bags ($200 ish), are stiffer, and look way better than the Zipps. Plus, Garmin Cervelo rides Mavic which will make you look even better.
The Mavics also have much better skewers.
Zipps are supposedly better because there aerodynamics is slightly better. They are not better. For those who disagree, ride both sets before you reply to me.
Mavics are much better for descending and criteriums.
Edit: You should test ride them if possible and people on the internet shouldn't be your deciding factor. Most of the people commenting probably don't own aero wheels and are basing their reccomendation on what other have on the internet. Although I speak the truth, you have no way of knowing.