It really depends on what you want and your budget.

If points/money are no object, then you should go with the Cosmo. You get at least a one room type upgrade for being Platinum, and two Wicked Spoon buffets. Plus you can use your Platinum card to cut the line at the Wicked Spoon. Cosmo treats Platinums as well as any Marriott-branded hotel I've ever stayed at.

With the Cosmo, you can also get access to Marquee depending on the night. If you want to go clubbing and have guys with you, this is a big benefit. (If you are an attractive female, you'll get into any club you want to go to for free regardless of where you stay.)

My Marriott choices for Vegas in order of preference would be Cosmo > Renaissance > JW Marriott > SpringHill