Almost no one I know was pleased with the MGM. I'd rule it out.
The Mirage propery has good quality accomodations, is center strip, and if you are walking around, is the the best choice of the three.
Personally, I liked the Mandalay Bay when I saw it for the first time in April. It seemed to be just a bit different than all the other casinos. I don't know the quality of the rooms. It is also located on the old Hacienda area, as south on the strip as you can go without being in the airport.
Make sure you check out the "internet specials" on the casinos, as they each have special deals way below rack rates when available. You've also ruled out a bunch of other properties, hopefully for good reasons.
[This message has been edited by NJDavid (edited 07-01-1999).]