I haven't sailed on RCCL but Celebrity has fabulous food. They have a reputation for making everything from scratch and serving really top-notch, gourmet fare. It's one of the things they're best known for as a cruise line.
I really haven't heard many good things about the food on RCCL.
I actually find Celebrity food to be very bland. I find they try to be really gourmet but I feel it is just a missed attempt (especially compared to Radisson).
I find RCCL food is quite good. Not all the choices are great, but they do have some excellent choices on the menu. If anything I feel RCCL's food as improved (at least my tastes) and Celebrity's has gotten more bland.
Recently, I moved back to RCCL after going continously on Radisson or Celebrity. On the Serenade, I felt it had to top one of my best cruises. The service was great and really like the new layout of the Windjammer. The only issues I found with the food was I didn't like the Ribs and Creme Brulee (but even Celebrity's ribs are not great).