Like Frank, I have been on both more than a few times. The two lines are very similar in most ways. RCL megas have more / better high energy activities for young people -- rock climbing, ice skating, miniature golf. Celebrity has slightly better food in the main dining room, though the buffets are about the same. Celebrity is a bit more formal, with a number of small touches like welcome aboard champagne, live music during dinner, an escort to your room on embarcation, informal dinner with waiter service rather than a buffet, and butlers in suites. The Celebrity M-Class ships have superb alternative restaurants, though the C-Class and Zenith do not. The balance of both lines (spa, gym, internet cafe, library, bars, discos, show lounge, entertainment, pool, etc) are indistinguishable in my opinion, though Celebrity decor is a little more low key than RCL.
You will find a slightly older crowd on Celebrity, but still a very wide range of ages and circumstances.
Both lines offer a good product, and you will not make a mistake booking either one. Which ever offers the itinerary and price that appeal to you should get the nod.
Ted
