The Little Mermaid rooms are far more newer and also much brighter than the rooms at the Pop, especially the bath rooms, theming is FAR stronger than in the Pop, where theming in the guest rooms is basically reduced to a poster on the wall. We stayed at the AoA in 2012 for 10 days (and even more often at the Pop) and although the Little Mermaid rooms are a slightly longer walk than most (not all!) rooms at the Pop I definitely liked the AoA rooms more than those at the Pop. The Big Blue Pool, a fenced pool with access only for AoA resort guests, is much better than any pool at the Pop and the theming, especially the Cars area, is superior to any other value resort. Especially with children IMHO the AoA beats the Pop in nearly every way, even if you are staying "only" at the Little Mermaid rooms. The menu at the food court is also a little different than at the Pop, however this is a matter of taste, I preferred the menu at the Pop in this case.
Take the AoA.