I previously saw the mus.i.c show at the MGM Grand and wasn't sure if there was going to be a big difference compared to the show at Monte Carlo. But since I enjoyed the show so much the last time, I decided to go watch it again. If you get tickets early, you can probably find some discount through the Monte Carlo website, I got my tickets for 30% off. I definitely recommend sitting in the main floor close to the center aisle so that you get a full view of everything that's going on.
The show is a bit different from the show at the MGM, the production value is higher so the costumes and the set got an upgrade. If you've seen the show at the MGM, the story line is about the same but they added some new routines and the best part, they took off their masks at the end =), not sure if they are doing that at the end if every show or what but they didn't do that before.
The Monte Carlo theatre is bigger and the seats are like the movie theater seats (all individual). At the Hollywood theatre at the MGM, some seats were in a booth and they packed 6 people into one tiny booth and it was a uncomfortable but the Hollywood theatre was smaller so it felt more intimate there. But definitely check this show out =), the tickets are very reasonable for a Vegas show, they even have meet and greet passes but no discount for those =(.