For our first cruise, i put location as a priority and the ship experience was horrible for me. For our second cruise, we chose the ship first and the port locations were secondary. we enjoyed the cruise much more.
This time though, we didn't want to go back to the same locations as the other 2 cruises and were considering going through the Panama Canal. Doing this, limits the ships we have to choose from. For Celebrity, it's Millenium, Infinity or Mercury.
My best advice is to weigh EVERYTHING...
First, we are all different...and we cruise for different reasons...
For me, oftentimes the itinerary is paramount...I cruise to see new and different places...Other people cruise to catch some sun, lie out at the pool and be pampered...So, for them, maybe the itinerary doesn't matter...
But, once I have an idea as to where I want to go, I consider the cruise line, the ship and the "deal", probably in that order...
I look first by cruise line...I always hope Celebrity is cruising somewhere I want to go on the dates I have available...If not, I will look at Oceania or Azamara even though they cost a bit more--there are some great itineraries and I will pay more to get what I want...But, aside from itinerary, I prefer to have excellent service and an uncrowded ship...Celebrity gives me this at the best rates for a larger ship--the best value...Oceania gives me that but in a small ship and a higher price...but, you get what you pay for...
After that, I'll look to Royal Caribbean--there newer ships are outstanding in terms of design and amenities...
I've yet to have a great experience on Princess, dislike NCL and absolutely abhor Carnival (tacky, crowded, poor service)...so those would be, to me, a last resort...
I enjoy cruising on the newer ships (though I know people who, for whatever reason, prefer older ships)...I like the amenities and the improvements they keep coming up with...Not...my next three cruises are all on brand new ships: Celebrity Eclipse, RCCL Allure of the Seas, Oceania Marina...
What limits you from cruising most of the newer ships is size...At present, the Celebrity M-Class is the largest ship that can pass through the Panama Canal...When they brought the RCCL Mariner of the Seas to Los Angeles (and when they send it to the Mediterranean next year), they had to route it all the way around South America...
As to those three Celebrity ships, they are sending them to Alaska for the Summer season and back to the Caribbean for winter...hence those Canal voyages are what they often call "repositioning" cruises...
The Mercury is an older, smaller ship, so, were it me, I would stick to one of the two M-class--Millennium or Infinity...neither is brand new anymore, but they are great design...over 45 gross tons per passenger--very roomy (Celebrity blows away most of the other big ship lines in this regard)...
I would not worry about repeating a class of ships, especially if you enjoyed it the first time..."Different" may not mean "better"...
Good luck...
