You are definately going to get alot of different opinions on this post, which isd I'm sure going to be very helpful, or very confusing for you.
I personally liked Carnival over Royal Caribbean, but then again that might have had something to do with the ship itself. We cruised on Carnival's "Victory" which was a much newer ship in comparison to RCCL's "Radiance of the Seas" in which I was not impressed with. I did notice that Royal Caribbean seemed to have had better food, although the entertainment was nothing in comparison to Carnivals!
We are scheduled on Carnival next by choice!!
My advice to you would be to try one this year, and do your comparison cruise in another year or two