I have taken cruises on both ships in question.
First of all, there is no "bad choice" among them. They both offer a good, mainstream cruise product.
Personally, I did prefer "Carnival Liberty" for the simple reason that I found the food and service onboard to be better than onboard "Jewel of the Seas". Also, "Carnival Liberty" has larger standard cabins and the passenger flow is better IMHO because "Jewel of the Seas" lacks another stairtower.
"Jewel of the Seas" is the prettier ship from the outside and has a better connection to the sea as far as the interior spaces are concerned (lots of huge
windows everywhere. But once outside, I prefered "Carnival Liberty's" unobstructed-view teakwood decks to "Jewel of the Seas'" Bolideck and Astroturf decks with most forward views "filtered" by glass. "Jewel of the Seas" has a superb accessible bow, however.
Whatever you decide, I'm sure you will be making a good choice.