Those are my two favorite places in Vegas and about the only ones I give a decent amount of action.
Lately - the last year or so - Cosmo has been significantly more aggressive (at least for me) with comps/offers/etc. So, if comps are your top priority, Cosmo is probably the better bet.
As far as comparing properties...both are great IMO. Really depends on your tastes.
Cosmo has those great balconies going for it. And the location can't be beat. The "young vibe" cuts both ways for me. More often than not, it adds to the fun and excitement. But, occasionally, huge groups of overserved kids can be a bit hard to take. There are some great bars and restaurants and the proximity to
Aria and
Bellagio adds tons more options. Gambling - with the exception of pretty terrible BJ - isn't too bad for a top tier strip property.
For me, Wynn is the best all around place in Vegas. The rooms are great, the service is far above that of the other mega resorts and everything is always spotless. There are some great bars at Wynncore - though a completely different vibe than the ones at Cosmo to be sure - as well as plenty of top notch dining options. And with V/P next door, you don't have to go far for a bunch more. But, they don't comp drinks on bartop VP and they can definitely be a bit stingy on the comps. (And if you're a craps player, they only offer 2X odds.)
Again, really depends on what you're looking for...hard to go wrong with either, though.
