Wonder how you wound up with these two - guess you are looking for ship out of Miami to Caribbean - ???

Epic is bigger - Epic (2010) is newer ship in terms of original build - but Carnival Liberty was pretty thoroughly redone in 2011.

It all depends on what you want in a whole lot of ways. Price and itinerary usually answer a lot of questions. If you are going to get a suite, then I hear from my rich travel agent (HA!) that the Haven on Epic is as good as it gets.

I have been biased against NCL simply because of particular anecdotal reports from family and friends that go back many years now - no real basis probably. I think the current reality probably is that NCL and Carnival are both mass-market lines that are more similar than different. You probably will have a great time either way. Again, price and itinerary are normally the primary considerations.

It probably would make sense if you want to get someone who knows the industry to help steer you to what could be your best bet, you should call someone like Bonnie Buchanan (who is TTOL sponsor and you can find her that way). Using a TA does not increase the cost of the cruise from the cruise line. Without knowing a lot more about you, even though we have cruised a lot, I would feel completely unable to guess which ship might be a better ship for you to pick. A "pro" for one person could be a "con" for another.