What's most important to you? Celebrity and RCI share the same touring outfit in Alaska and I'm sure the prices and options are similar.
Mercury and Vision are two different ships. Though similar size and vintage they have different features and ships have different styles. The service style of the lines are different. Celebrity tends to be more formal, RCI more active. Celebrity is quieter of two lines.
I like Celebrity for the lack of public announcements, overall clean and very well maintained and understated art-deco inspired ships, food presentation, fairly unobtrusive service, passenger space ratio, crew to passenger space ratio, and overall quasi-traditional nature of the marque. I also like that the ships lack the RCI trademark centrum and have more modest and intimate foyers. Celebrity is also known for the very extensive spas and their cruisine that many feel is not what it once was. What some point to are weaknesses, but I don't mind at all, is the fairly rigid hours of food availability, pool and hot tub availability, quiet pool times, and not so extensive alternative dining.
Vision of the Seas is aptly named. It is a ship with lots of (teal tinted) glass, more tropical Hyatt-like in environment overall, has the centrum (multi storied atrium with glass elevators) as a heart of the ship with varied activities, has more banter and interactivity between the service staff and the pax (ship's passengers), I think has mini-golf and has more extensive alternative dining and an overall more casual feel. I'm going on the Explorer of the Seas in 2 weeks, my knowledge of RCI is through feedback and research, so it'll be my first cruise on RCI. I have a different mind set going into this cruise than I have when I sail Celebrity and I will not bring my tux or some of my dressier clothers which I bring when I sail Celebrity. I'm thinking of RCI as a more casual and more upbeat environment.