First, talk to a reputable cruise specialist who can help answer all your questions so you have the necessary information to help you decide on what's best for you.
We've been on both of these ships and they are completely different. For example, the Splendor is 70,000 gross tons and was built in 1996. The Liberty is 160,000 gross tons and was built in 2007. So the experience you'll get is completely different as well. The children will no doubt love the Liberty more because it has alot more amenities, whereas the Splendor, by comparison, does not really have alot to offer children.
As for the itineraries, this is really a personal choice because it depends on what you want, where you like to go, and what you're looking for in a cruise. Both of these offer something wonderful, but are different in what they offer.
We were on the Splendor out of Venice - did two 7-night cruises back-to-back, so we did the itinerary you're looking at. It was absolutely wonderful and we thoroughly loved it! Weren't real crazy about the ship, having spent some 57 nights on Vision-class ships, but we didn't do the itinerary for the ship, we did it for the adventure. But I could tell you great things about each of these amazing ports as we loved all of them.
Tough decision to make, so talk to your agent, get all the information you can, and just go with the one you think is best for you.