Hello- We are trying to decide which cruise will be better for our winter vacation. Please help us

Navigator of the Seas(4 day, balcony but only stops 1 port;Cozmel)
Celebrity Century (4 day, Concierge balcony but smaller older ship? stops 2 ports)
We have never been on the Celebrity Cruise line before but we have been on several RCI including Mariner of the Seas. Century looks nice from the pictures.
Oh- and one more question! Is it better to book front or back of the ship? We just got back from SOS and our room was towards the front of the ship... I got sea sick from all the rocking!!
Any help would be appreciated!!