I've sailed Century, Galaxy, and Constellation. I've toured the Millennium, Mercury and have spent many days onboard Zenth and Horizon when they were in port in New York City.
I'd pick Century over Millennium, but my picks are based on my biases and priorities for ships. I don't go for the biggest ship nor do I go for the newest. These are irrelevant almost to me as I feel a great ship is made from inspiration and standards and there are times where there is more inspiration and higher standards than other times. I feel the quality of the Century is better, better quality of finish (it was made in a German yard where the Millennium was made in a French yard. The German yard is known for producing ships of higher quality). I feel, per size of the ships, there is more life, closeness, intimacy, and friendlessness onboard the Century than there is the Millennium. The Millennium, IMO, is a ship that has too few lounges and overall not a lot of public spaces per the overall size of the ship. And that translates in less options for passengers. It is an absolute misconception that some people have that the larger ship means more options or more to do for passengers, that is not always the case. I found more venues and things to do on the Galaxy and Century than I found on the Consetellation and I also found the Galaxy and Century more interesting as ships than the Millennium and Constellation. Galaxy and Century feel like two different personalities of Celebrity and like two distinct ships. They really don't repeat each other in major areas. Millennium and Constellation feel like cheap carbon copies of each. While the ship's furnishings don't feel cheap, the absolute repetition of furnishings and placement of furnishings isn't very interesting, IMO. It feels like what it is. Celebrity saved money by buying furnishings in bulk, and they basically change the patterns and colors from Millennium ship to Millennium ship. Celebrity, when making Century, Galaxy, and Mercury, bought custom furnishing for each ship, each foyer, dining room, major lounge is different and unique per ship, and the results feel like more personalized ships than commodity ships. They also had their team of 4 or 5 designers switch areas for the different Century ships, also helping the ships have different personalities from each other.
I like ships for social venues, specifically lounges. In the Century, you have 3 venues where you can hear music, dance, and get drinks. In the Millennium, you have 3 as well, up from the original 2. What happens is that Millennium's cigar bar proved oversized and underused and the space was transformed into a piano bar. So, there is piano bar entertainment, however, no dance floor in the lounge. On the Galaxy or Century (or Mercury, Zenith, or any other pre-Millennium ship), you can dance in the aft nightclub to whatever music is playing there or just hang out and enjoy that music and atmosphere, or the Rendezvous lounge that usually featured a duo, or the upstairs observation lounge, and still you had areas where you could hear a harpist of piano player in the foyer or in Michael's Club. On the Millennium ships, there is no aft lounge, there's an area above the Rendezvous lounge that has an overlook, an actual hole in the floor, down to the Rendezvous. The problem with this is the two areas share the same music. Essentially, there is one less venue for music or dancing, this area isn't separated from the Rendezvous, and one hub of activity and mingling area is reduced in the bigger ship. I thought this was a poor design decision on Celebrity's part.
There are other aspects of the Millennium ships I find underwhelming. In the main dining room, on both levels, there are long wings that lead to the main part of the dining room. Diners in this area feel apart from the other diners. I feel this in much of the ship, there are many areas that feel like corridors with seats rather than friendly conversation areas that are part of a larger space which allow passengers to feel like that are part of what's going on in the area. That's why I find the Century ships to feel more comfortable and friendly than the Millennium ships. I find the thellasotherapy pool much nicer on the Century ships. It is a smaller, more private and special feeling area with strong jets. On the Millennium the jets are weaker, it feels more public, it is noisier, and maybe that is why it is free (sometimes there are problems with children using the area as well, this is not an issue for the reserved t-pool on the Century ships). Also, the Millennium ships needed to redesign some areas since they were put in service. The observation lounge was too large and not separated into different areas as they are in the Century ships, and were underused. Celebrity has repurposed and segmented an area for the lounge for Cirque de Soleil balls to the Constellation and Summit. If they prove popular all the M ships will get them. They also will get rid of Notes, a two-level listening library that hasn't proved popular. There's also the conservatories. Here's where Millennium is different. Her conservatory is small and forward. She has a sports bar. I find that a much-needed lounge on a ship that really doesn't have that many. Unfortunately, IMO, the Infinity, Summit, and Constellation have large plant shops called conservatories in that area. Also, I think the Millennium is the most elegant of the 4 ships and I can point out specific areas of the ship where it is noticeable compared to her sisters. I find her specialty dining room to be the most elegant, and this is the one area where the four ships are not carbon copies of each other. Of the four Millennium ships, I think the Millennium is the most desirable and most elegantly appointed. She's not the pretties, perhaps the Constellation is due to its more tropical colors, but Connie gets overdone in certain areas, IMO.
In terms of deck space, again Century is better. They are more varied, there's a small area in front of the lido that was wonderful and very much used when I sailed the Galaxy in Alaska. You can walk around the back of the Century's promenade where the Millennium only has the promenade deck on the side of the ship and no wraparound to the back of the ship. There's teak on Century's promenade where there is a rather impersonal nonstick flooring on the Millennium ships. There's acess to the back if you are interested where this area is all personal balconies on the Millennium. I'm not big on balconies myself, I like different levels of deck space and Century was designed before the balcony craze.
For me, if you need the balconies, specialty restaurant, somewhat better passenger space ratio (but keep in mind that the passenger ratio is a range--from 2000 or so with double occupancy all the way up to 2450 if all berths are full), then the Millennium is the pick. Besides that, I don't see where she offers anything over the Century unless you count that she is 5 years newer. There is also the light-fare food in the aqua spa. I've read as many complaints about the Millennium feeling worn as the Century, hopefully either one you pick would be as well maintained and clean as Celebrity is capable of. When they are on, Celebrity has some of the cleanest, best maintained ships in the industry, but this has unfortunately slipped the past few years.
Most of Celebrity's major signature areas were introduced by the Century and the Millennium basically has larger (stretched) versions of them. These are aqua spa, Cova cafe, Tastings, Michael's Club, provocative art, 2-tier aft located dining rooms with large picture windows ( I think the Century has Celebrity's absolutely most elegant dining room, and it is unique--no other Celebrity ship shares her dining room)
In my opinion, while fine ships in terms of decor, the newest Millennium class Celebrity ships don't measure up favorably to the more completely thought out and better-finished Century ships.
