I've stayed at both, I personally prefer AoA.
Room sizes are pretty much the same if you are going to stay in the Little Mermaid rooms at AoA. The room theming is much better at AoA than ASM, although both grounds are amazingly themed. The rooms in ASM are looking a little tired now compared to the fairly new AoA.
The little mermaid rooms are a fair hike from the food court and bus stops at AoA so if being close to them is a priority, I'd consider spending the extra money on a preferred room at ASM. We found the buses to be equally good at both hotels and never waited more than about 15 minutes, even when they stopped at every all star resort.
The pools at AoA are definitely better than ASM, as is the food court and the location of AoA is better as it is closer to most of the parks and it is next to hourglass lake which is really nice to walk around.
Staff at both resorts were amazing and you'll have an amazing time whichever resort you choose! Have fun!