I've saild on RCL and Celebrity. From my experience, Celebrity is my choice. RCL seemed to be geared more to families, with a good number of children on board. Also, the food was not particularly exciting, and I wasn't impressed by the dancing and singing waiters, and the theme nights (color coordinated napkins??) and parade of flaming baked alaska (or whatever it was). The constant announcements on the public address system are annoying.
Celebrity is a cut above. The food was much better than RCL. The average age is older, but I found interesting people, and plenty to do. I still correspond with passengers I met on Celebrity four years ago.
I am 47, and my partner is 48. We are booked on Celebrity Mercury (his first cruise since being in the navy) sailing on 1/6/01, Rio to Valparaiso, around Cape Horn. We are very excited about our upcoming sailing.