I have mostly sailed Celebrity for the last ten years; however, sailed on Mariner of the Seas this past August. I made the switch because RCCL's itinerary to Bermuda worked best for our group. I would sail RCCL again if the itinerary worked. My next four cruises are Celebrity.
While I am not a "foodie", I did find Celebrity's food to be better. Portions on RCCL in the MDR were smaller (which was probably a good thing), but the flavor on some of the classic selections is better IMO on Celebrity.
We booked balcony cabins on RCCL and follow suit on Celebrity. The SV is my balcony of choice and I did that on two Edge sailings.
I am Elite on Celebrity which transferred to Diamond on RCCL. As a non-drinker, I shared my four daily drink coupons with my family.
I love that I now get a free bag of laundry on Celebrity...didn't get that benefit on RCCL.