I believe that my experience was isolated as I have found wonderful reviews of the Sovereign of the Seas, but I will tell you of my expereince. I will start by saying that I work in the Tourist industry and I am a firm believer that for every 1,000 people that has a great time one does not... I was the one
We are hige fans of RCCL and Disney, these are the 2 cruise lines we like to sail (15 crusies and counting between the two). Our experience with Sovereign was not a good one. We found the ship to be worn down (I know it's older and expect that) the rooms were VERY small compared to Disney and some of the newer RCCL ships (again I expected that due to it's age). However we had horrible service. I was yelled at by our room steward because I was in the room and did not have the Do Not Disturb sign out... heaven forbid I answered the door when he knocked. The Purser's desk messed up our charges and after 4 trips to them to fix the problems I was assured by the manager that it was taken care of, only to find out 2 weeks after we returned home that it was not and then battled with them for another month before it was finally fixed. We had some other issues in Nassau which I won't go into huge detail (that would take forever) about but the crew would not let my husband back onto the ship because they said he needed his parents with him it took us forever to explain to them that we were married and that even though he was under 25 he did not need his parents because he was with me (I was over 25)...not that that would be an issue for you, but it just goes in my poor service experience.
The Majesty is the newer of the two with her maiden voyage being in 1992 were the Sovereign's madien voyage was 1988.
They are not "party" ships, I think that your mother will enjoy them. There is quite a bit to do on board so there won't be a lack of activites but it is not a "party" ship. Entertainment is good. As for the food, I have to say that it is the best and the service in the dining room was the highlight of our trip...
You can look at Mytravelco.com (on the main page there is a link on the left that says 360 degree ship tours, from there it brings you to all the cruise lines and ships) there are links to view the rooms so at least you will have a good idea what you are getting before you arrive.
Anyway, if it were my choice I would go on the Majesty, only because I have had a poor expereince with the Sovereign (which again I believe my experience to be an isolated incident as many others love the ship and rave about it).