If the price and itineraries are the same this will come down to the ship differences obviously. Big difference in the layouts, I prefer the Legend's layout for ease of getting around to the Conquest class of which the Liberty is one.
The Liberty has the Seaside Theater, and the Legend doesn't. The Liberty also has a Volley Ball court.
The specialty food outlets vary greatly between the two ships, some of the differences are;
The Liberty has a Mexican outlet, as well as Fish n' chips, Mongolian Wok, and Outdoor BBQ.
The Legend has a Rotisserie and an Asian Delights outlet.
Anyway, I think that is at least some of the differences. Personally, I'd be glad to go on either ship. The Seaside Theater is a consideration to some cruisers. Also, the ship sizes and guest capacity. With the Legend being 88,500 tons and carrying an approx. 2124 passengers based on double occupancy as opposed to the Liberty at 110,000 tons and carrying approx. 2978 passengers. So the less crowded feel of the Legend comes into play as well with some cruisers.
is a link to the Legend's deck plans.
is a link to the Liberty's deck plans.
Hope this helps some.