There might be construction at MC, they are doing that huge stadium and also alot of remodels.

Aria still has nice rooms.

Depends on your needs.

The suite at MC will offer a bigger room and couch options.

 
I'd go with cheapest option and use the money you save to see a nice lady from TER.

Very nice rooms granted if you have company you will have to come down since there is security at the elevators.Monte Carlo does not have that problem.
However like another reply mentioned there is construction since the new stadium is being built right next door.Also I think in the Monte Carlo suite there is a big jacuzzi tub in the bathroom that is pretty fun
Enjoy and have fun at whatever property you pick

Stayed at both several times.  The construction is not an issue at MC.  If a Diamond Suite is in your budget definitely stay at MC.  Wet bar, fridge, separate bedroom, huge tub no security.  Cheaper food options.  The Aria is nicer and much more upscale.  24 hour security.  The spa suite at MC is about the same price as a regular r[m at ARIA.  Larger open room, fridge, giant tub.  I stay at MC and gamble at ARIA since they are connected.