Everyone chooses their cruise for different reasons. We tend to pick the itinerary first. Just because the Jewel and the Dawn are older ships doesn't make them bad choices. Take each of the three and list what each has to offer in the way of on board amenities and their itineraries, and decide what appeals to you the most. You may have to do a little compromising if you can't get everything you want in one cruise. But that will give you a good excuse to go on another one.
Carnival Liberty May 2013
Carnival Liberty May 2012
Carnival Sensation Feb. 2008
Carnival Destiny Feb. 2007
RCCL Explorer of the Seas March 2002
RCCL Majesty of the Seas Dec. 2000
Carnival Imagination Sept. 1999
RCCL Enchantment of the Seas Feb. 1999
RCCL Grandeur of the Seas Feb. 1997
What happens on the ship stays on the ship, unless it gets posted on YouTube.