Zipp and lighter too.
I´ve only ridden the clincher models and can say that in that area Cosmic outperforms Zipp. Weight is almost the same for clinchers. Cosmics are stiffer, and more durable since the spokes are connected to the alu rims on cosmic and to the carbon rim on Zipp. My Zipps loosened at the alu rim/carbon area. I´ve read that the area where the spokes are connected to the rims on Zipps may crack easily (never happened to me).
When it comes to tubular Mavic has released cosmic for tubulars but here Zipp wins by far beeing much lighter ( I say this as an opinion having never ridden the tubular models), also since you can buy Zipp rims you can build with other hubs than standard Zipp hubs and reach an "ideal" setup. So if you looking to buy a tubular wheelset (and dont have the cash for lightweight) Zipp seems like a good option.
I comes down to what you´re going to use it for. I live in a flat country where the weight difference is less an issue. I like to "cheat" some times and use the wheels for training. They feel solid, they were "cheap" compared to other "carbon" wheels. So I use them a lot and I dont really worry about them braking (yet). In races they work well, since climbs are short. And the hubs roll really great so I´m a happy customer so far. I´ve owned Zipp 303, 404 and now have the Carbone SL
If you live in a hilly place and are going to use them solely for racing Zipp tubulars would be a good choice (model depending on your weight and riding enviroment) And there are several other brands you might want to look at too like Corima, Campagnolo even Shimano.
If you are looking at clinchers I would get the Mavics as opposed to the Zipps. Mavics are more solid,weigh the same, are much cheaper.