Both Celebrity and Royal Caribbean are fantastic cruise lines.
Celebrity is our favorite cruise line but is more reserved.
Royal Caribbean is more laid back and seems to cater to the younger generation (not that Celebrity is for the older folks). You will find on each cruise line the fun that you make of it.
Since it is your honeymoon you will enjoy either line. No one can spoil the moment of the line and excursions and cruisers you will meet on the honeymoon.
You have sailed Carnival and giving another cruise line a chance is commendable.
If I read between the lines Royal Caribbean might be a better choice.
Think now about the itinerary and ports of call. Do you wish the pampered side? How about the cabin being booked, balcony, suite etc?
The Millennium is about 1950 passengers, have not been on the Mariner but have been on the Voyager, over 3000 passengers and did not feel crowded.
To the nitty gritty, Millenium will give you a more intimate cruise as Mariner might be the fun you are looking for in a honeymoon cruise.
The best thing to do is to continue and ask questions here on the message boards.
We here at CruiseMates are here to help.
Round trip Los Angeles to Hawaii, Summit March 26, 2006
Vancouver _ Alaska _ Los Angeles, Sept 16, 2006 on Summit