I am a Platinum member on Disney and a Diamond member on Royal Caribbean. I feel that I am relatively objective, and enjoy both lines. I also have few financial constraints (other than the psychological restraint of not wanting to throw money out the window needlessly). I just checked prices for three ships on the same date in April of 2023 on similar Caribbean cruises -- the Disney Fantasy, Oasis of the Seas, and Wonder of the Seas (RC's brand new ship). I priced for two adults and two kids ages 10 and 12. The results for an ocean view balcony/veranda: Oasis of the Seas-- $3172; Wonder of the Seas -- $6,912; Fantasy -- $9,566. The Junior Suite on the Wonder of the Seas is $8,642. Thus, you could add the ultimate dining package to your Wonder of the Seas cruise which allows the entire family to eat in all restaurants for about the same price as the Fantasy in an oceanview with veranda. You could sale on the Oasis for a third of the cost of the Fantasy. Both the Wonder of the Seas and the Fantasy dock in Port Canaveral (the Oasis is in Miami for this cruise).
The Oasis of the Seas is a great ship for teens. It has far more teen activities than the Fantasy, and goes to RC's private island that is far better for teens than Castaway Cay. The Oasis is similar to the Fantasy in terms of kids waterplay areas. The shows are better for kids on the Fantasy, but not for teens. The Fantasy is better for kids under 10, but I cannot really see it being better than the Oasis for older kids.
If you compare the Junior Suite with UDP on the Wonder with a standard cabin on the Fantasy, you are comparing apples to apples on price. But, unless you really, really love Disney, the Wonder of the Seas is a better option (unless you have small children).
Small kids: Wonder of the Seas is the winner. The Wonder, like the Fantasy, offers childcare for the 3 and under set. There is a great splash pad for kids in diapers. There is a playground for little ones. The only Disney advantage is characters, and I'm not sure the under 3 set cares.
3-10 year olds -- The Fantasy wins in my book for the 3-10 year old group, Disney wins simply because it has shows and characters that are a better fit for this age group. It also has the games and other activities that have a slight edge over the Wonder. Wonder of the Seas has a carousel and play structure, but if you have a kid that likes Disney, it wins.
Wonder by far for the 10+ crowd -- For families with older kids, there is no comparison; the Wonder of the Seas is the clear winner. First, you get a larger room. You get access to the Suite restaurant for dinner with no upcharge. With the UDP, you get access to a 8 specialty restaurants that offer an amazing range of dining experience. The pool area for older kids is better on the Wonder. The water slides are better on the Wonder. The Wonder has dry slides, rock-climbing walls, a flowrider (simulated surfing), ice skating, all kinds of sports activities, a great teen gathering place, and entertainment better suited for older kids. RC's private island is far superior to Castaway Cay, particularly if you buy waterpark tickets.
Having sailed on both lines on many different ships, once my kids got older, I could no longer justify to myself the higher price of Disney. I have never experienced more "upselling" on RC than Disney ships (but perhaps that is just me). The additional cost has always been about equal for us regardless of ship or cruise line. I will go back to Disney when my two grandkids are in the 4-10 age range for the reasons stated, but with younger or older kids, I would stick with RC.