During most of the year the crowd with be much older on Celebrity. I think the only exception may be if they have a sale during spring break or something. I expected it to be much fancier than RCCL but what I experienced was that it was more of a moderate elegance as opposed to being updated and modern. When I cruised Celebrity it was among the smaller ships I've cruised and there was NOT much to do if you're an active cruiser. DW enjoyed the slow pace and she could sit in a lounge chair and stare at the ocean all day. Me? I have to have something to do, at least every now and then. On my ship it seemed like the only things to do were gamble, dance, swim, shop, or attempt the windiest game of basketball ever. Far, Far fewer options than RCCL, which is aimed at the more active and younger crowd. There are not as many loud obnoxious pool games or parties popping up everywhere, although that isn't to say they didn't exist.
I don't meant to make it sound like cruising Celebrity is akin to visiting a luxury retirement home, but there were times when it had that pace... Full disclosure, the oldest/slowest crowd DW and I ever cruised with was actually on RCCL 9-night in early December before school let out for most children and it was one of the best ever. No lines for the rock-climbing wall or pretty much anything. You could do whatever you want except for 'walk down the hall quickly' and ' get a pool deck chair at 7AM' (fine with me). Felt like we had run of the ship in the late afternoon and evenings, but so many events were cancelled because of lack of participation.
It looks like some of Celebrity's newer larger ships have incorporated more offerings, but it seems that they stick to their bread and butter 'relaxation' theme (spas, croquet, slower paced activities) and there is NOTHING wrong with that.
I firmly believe that any cruise can be a great cruise, but I still have my preferences.