Here are a few ship facts for you on the Milennium....
Celebrity Millennium:
Ship Facts
Occupancy: 2,034
Tonnage: 91,000
Length: 965 ft
Beam: 105 ft
Draught: 26 ft
Cruise Speed: 24 kts
Inaugural Date: Jun 17, 2000
Celebrity is actually owned by the same company as Royal Caribbean (as is Azamara cruises) and you cannot go wrong with either line. Celebrity is a little bit more upscale. That said, I was on the Mariner of the seas in Sept.2007. We were in a balcony room on deck 7 and I cannot say enough great things about this ship. The service was excellent, the ship was fantastic and the entertainment onboard was wonderful. I don't think you can go wrong with either ship.
Rates right now for the Mariner are really good and if you can get Maui out of the trip as well that would be my pick. You really can't go wrong with either though