Just stuff I have gleaned from reading on here:
If you are in the Lion King building, try not to be on the 1st floor, because people will sometimes use it as a throughway on the way to Little Mermaid (though I bet that happens most in the summer, not January). Nemo is "closer" but if you look at the map, it really depends the most on your room placement. A further Nemo room is going to be less convenient than a nearby Lion King. The Nemo rooms seem to be the most popular and thus hardest to get discounts on (and seem to sell out, just in the week we're looking at). I would just pick what room and design you like best. Personally, Nemo is my favorite movie and I like the colors and design in that room best. The Cars are kind of blah to me and Lion King is too loud. But again, that is totally personal preference. I don't think you can go wrong either way! I'm so excited to go!