With the Vegas Marriott hotels, it really depends on what you want.

Cosmopolitan is right on the strip, has great rooms, a delicious free buffet (if you are Platinum), and free access to an awesome club (a big benefit if you are a guy). It attracts lots of younger people with rich parents, and a little black dress/nice dress clothes is pretty much the cost of admission (lots of people showing off). The downside is that it is 35,000 points/night. I'm at 100% for upgrades, so 35,000 points has been worth it to me.

If you don't care about being on the strip and want something more quiet/relaxed, the JW Marriott is a great choice. It's about a 15-minute drive. I usually get some sort of Platinum upgrade here. It attracts an older/more affluent crowd for the hotel portion. The attached casino is old and smoky, but I enjoy it. If you play a bit, you'll get meals comped at the casino restaurant, which is actually quite delicious if you aren't a food snob. They stopped doing free breakfast for Platinums a while back, so you'll have to earn your breakfast in the casino. The pool here is very nice.

The Renaissance is a nice hotel. It's about 5 minutes off of the strip and next to the convention center. Hotel restaurant is excellent, and Platinum upgrades are nice here as well. The staff have always been very nice to me. This hotel has a concierge lounge, which the JW and Cosmo do not have. Last time I stayed here they provided free breakfast for Platinums, but I'm not sure if they still do. The downsides to the Renaissance are the small pool (which allowed smoking last time I was there) and the location (unless you have a rental car or are attending a convention, which makes the location a non-issue).

Hopefully this will help you in making your decision. You can't go wrong with any of these hotels, but you should pick the one that best matches what you are most interested in.