I have my first ever trip to Las Vegas coming up with the family (ACM awards). I struggled a bit with this same question and went back and forth a bit on each one. I've finally decided on the Grand Chateau because it was less points, no parking charge, more room and no resort fee. So basically I could pay about $500 for my "free" room at the Cosmo or virtually nothing at the Grand Chateau.

Also, if you look at the main picture of the Grand Chateau on its Marriott website, you will notice the Cosmo right in the middle of the picture too, virtually just across the street.