That's quite a choice, very very different atmospheres and menus...totally!
Narcoossee's was once a very good restaurant but lately I've heard a lot of mixed reviews. It is known for it's seafood but does have a few other options. It is rather pricey. It is a casual restaurant but it is in a nice location.
LTT of course in the MK and has the characters which makes it lots of fun. The food is totally different, typical American fare, all you can eat but served family style. The food is good there as well. The price is also probably a better value for what you get.
It just depends on what you're looking for. Narcoossee's tends to be more intimate. LTT has more of a hussle bussle. The only thing they both have in common is their dark atmosphere as the lights are pretty dimmed at both places.
If you plan to be in the park then go for LTT otherwise Narcoossee's may be the better choice.