my $0.02, since I have a pair of each:
Ultegra on OPs, 32 spoke (butted), 3x build. Durable, reasonably lightweight, easy to maintain, great for training, good enough to race.

Mavic Cosmos: Not a lot of experience, I've got maybe 600 miles on mine. I got a pair after harping forever on the benefits of Ultegra or DA on OPs in other threads. They're durable, about the same weight as Ultegra on OPs, a few less spokes. I can't really tell a difference in spoke wind resistance "at speed." But I've never really tried to measure it. From what I've read, they are durable enough to train on and race with (the rim is basically an OP). The hubs have more friction than my Ultegras, but again, I don't notice it. A pair will cost about $100 more than Ultegra/OPs.

I guess for the money vs. benefits, I'd go with the the Ultegra/OPs.