I think $100 for Mandalay Bay for your dates is a very good deal given the close proximity to New Year's. In terms of quality, I'd agree with the others that the two top choices of those you mentioned would be MB and the Venetian. Most 21 year olds would probably prefer MB, as it's the hipper of the two.

If you were going during pool season, I would definately recommend staying at Hard Rock, if for no other reason than it would give you access to their pool scene. I needn't go into the details of what you'd find there, just trust me, it's *hot* <g>. Save Hard Rock for your next trip in warmer weather.

BTW if you stay at MB, their wave pool and lazy river won't be open then, although one smaller pool will be.