None of the list ships are that old. The Carnival Legend began service in August 2002. The Carnival Valor began service in December 2004. The Splendor has not begun service yet.
Since the Splendor is not out yet, I don't really know how it compares to other Carnival ship. However, I believe it is just going to be a bigger version of the Conquest class (the Valor is a Conquest class ship). Nonetheless, when looking at the deck plans I don't see a water slide, so it is hard to tell what the Splendor will be like when compared to the other ships. The deck plans do show that the Splendor will have the Sea Side Theater (a large screen TV for watching movies or sporting events while out by the pool).
Nonetheless, my favorite class of Carnival ship is the Spirit class. The Legend is a Spirit class ship. So, based on ship alone, I would pick the Legend.
If I were you, I would look to see which ports interest you the most, and pick the ship that goes to those ports.
Here is a photo of the Carnival Glory (which is what the Carnival Valor would look like) ...
Here is a picture of the Sea Side Theater ...
Here is a picture of the water slide on the Carnival Glory (it would be the same on the Carnival Valor) ...
And here is a picture of the Carnival Legend ...
