Having sailed RCL 5 times (2 future cruises booked) and Celebrity 4 times (with 4 future cruises booked) , my first preference is Celebrity.
I agree with hcat that the Concierge Lounge is a nice perk on RCL and I also wish they had something like that on Celebrity.
If I do cruise RCL, the only ship I would cruise from now on is the Radiance Class (Brilliance, Jewel of the Seas). These are RCL's most beautiful ships.
It's all a matter of what you want out of your cruise experience.