If you want to try something different then you need to go with the Sovereign. Personally I think it is am more interesting ship, and was recently refurbished with extensive changes made in the layout. For a 3-night cruise you really aren't going to see a lot of differences in the crowd between the two cruise lines. RCI is a tad more relaxed than Carnival although not terribly much.
A huge factor between these two cruises is the itinerary. It is kind of pointless to be in Nassau until 6am on the Sensation. On the other hand, the Sovereign leaves earlier and gives you the last day of the cruise at Coco Cay where you'll have a great beach party and barbecue.