Not a whole lot of difference. I have a Cosmos set on my training bike and they work fine; pretty bombproof. The Cosmos rim is the same as an Open Pro but with a slightly lower spoke count. One advantage of the Cosmos is the sealed bearings as opposed to loose bearings on Ultegras. The QRM sealed bearings in Cosmos hubs are very smooth; in fact I think they're the same as what's used in the Ksyriums. An advantage of the Open Pro/Ultegra is that if you're out somewhere and break a spoke, they're standard spokes and just about any shop can replace them on the spot; not necessarily so with Cosmos. But then again I don't really hear about people breaking spokes on Cosmos. The rears are x3 both sides.
If the Cosmos hubs are built like my Velomax/Easton Circuit hubs, believe they lose points on durability and serviceability vs the Ultegra. The Ultegra "cup & cone" bearing design is long proven for bicycles, with a easy way to adjust bearing preload, and a good sealing system to keep out contaminants.
The hubs using pressed-in bearings can have very low drag because they have no contact seals against the axle, and also no preload. They do have a non-contact face shield on the bearing itself, but those don't really keep out any water drops that gets past the hub end caps. The drag on any well-adjusted hub is going to be insignificant anyway compared to your BB and chain losses.
The cartridge bearings in the Circuit hubs are standard skateboard bearings, which are intended for radial-loads only. The Circuit hubs have no means to adjust preload. In order to get them properly set-up for low-drag smoothness, I had to add a shim of about .006 under one bearing on each hub. The Cosmos may have a way to adjust preload; if not, I'd consider that a disadvantage. Note the higher-priced Orion and Ascent Easton wheels do have self-adjusting preload via a wave spring.
One ride in a good rainstorm was enough to cause my Circuit front bearings to rust. When I called Velomax tech support, they advised that after riding in rain, it was best to pull the endcaps and remove any water droplets inside before they could rust the bearings. This only takes a few minutes, but not really something I want to do after finishing a long, rainy ride.
My LBS had to order the replacements in; even though they are a standard size (R6), not many LBS are going to have them in stock. Just like the spoke issue, standard parts always have advantages when you want repairs done on-the-spot.
Don't mean to come off sounding so negative. I'm still running my Velomax Circuit wheels at 12K, with only that one replacement to the front bearings after the rainstorm....and the shimming I mentioned. Depending on how and where you ride, Cosmos hubs may prove perfectly adequate for your use.