If you can cruise on either, choose the Liberty and the new Funship 2.0 refit over the Legend. I love the Spirit Class ship layout, but only a couple of ships in the fleet have the Funship 2.0 upgrade on them at this time and I think it tips the scales in favor of the Liberty.
Crowd wise, I doubt you will notice a big difference. The ship itself is bigger so there is more space and more venues for people to spread out in. So the extra people don't matter as much as they would on say the Victory where the ship is bigger but there are no extra buffet lines...
That's my input. Come on back when you have your next questions.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013