So much depends on what's important to you: ports, room size, food variety and quality and dining options, smoking/no smoking on board, casino, staff to guest ratio, entertainment, activities, demographic, sedate versus party atmosphere, set dining time versus dine anytime, spa, gym, price, time of year, excursion options, on and on.
We've only cruised Disney, but even when picking which DCL cruise, we take these factors into account. For example, I prefer taking one of the larger ships if travelling to the Caribbean or Bahamas since I'm not really too interested in the islands. If I haven't been to a particular island before, I might do a short excursion, but I'm more likely to stay on board and relax in the Cove Cafe or do some activities or go to the pool. So those amenities will be more important to me than the port selections.
I'm more likely to want pool time on a 7-day cruise than on a 4-day. After our recent cruise on the Wish, I can't say I'm impressed by the adult pool area. On a 7-day, that might be a deal-breaker for me, but on a 4-day, we spent about an hour in that area, and that was enough for us because there were so many other things to do and see. We probably spent more time on the Aqua Mouse than in the pool.
When going to Europe, we prefer the smaller ships so that you can get to more ports, fewer ports require tenders, and those that do are not as busy, there's enough on the ship to keep me occupied for the short time that I'm not in the port, and the rooms are a little larger than those on the Dream and Triton class which is important for storage because it's usually a longer cruise.
We've looked at other cruise lines and, for us, any cruise line that allows smoking in the interior is a no-go for us. That cuts out a lot of lines. Food is important to us, so any cruise line that doesn't have what we consider to be decent options in the main dining room will be a no-go because I don't feel like being forced to go to specialty restaurants for a decent steak or more than the average chain restaurant fare and to also have fight for bookings. I want a reasonably all-inclusive cruise where I can have a good experience without having to constantly fork out for things I think should be included (e.g., shows, movies on TV, room service, etc.) or where I'm constantly hounded to buy.
I don't need roller coasters and skating rinks (so probably not any mega Royal Caribbean ship), but I do want some decent entertainment options that aren't limited to nightclubs or Vegas-style shows and raunchy comedy (so not Virgin). I'm not looking for a party atmosphere, booze cruise, or one that caters to largely very young crowds (so not Carnival). But I do want a decent selection of higher-end spirits and wine. I'm also not looking for a retirement home at sea (so not Holland America). We may be in our late 50s/early 60s, but we're relatively active people, and we regularly weightlift and exercise, so I don't want excursion options that are limited to, or geared for, people who just want an easy stroll (so many river cruise lines are a no). And while we don't always go to the gym when on a cruise, I do want one available to me with reasonably good equipment, just in case. I'm not big on beaches or water sports so cruise lines that cater to that are not interesting to me (so no mega yacht or uber small cruise ships). This is the type of analysis of your own preferences and needs you have to do and then look at the different cruise lines and see which are a definite no for you. There are lots of videos from bloggers on YouTube as well as the cruise line website and promo videos.
So far, the only reason we've even been looking at other cruise lines beyond DCL is the ports. There are lots of other places I'd like to go, but it still has to be on my terms. IMO, a less expensive cruise line isn't a good value if it doesn't also provide you with the experience you want. So we start with the experience and then look at the price. If your requirements aren't as picky and rigid as ours, you may find many more options than we currently have found.
