I've stayed at both and POR is definitely my favorite! Yes, there is a $40 difference, but you will get SO much more! POR has 6 pools, ASMu has 2. POR has a food court and sit down restaurant, ASMu has a food court. POR has a huge store, ASMu's store is nice but much smaller. POR has boat and bike rentals, ASMu does not. POR has a beautiful landscaped property with lots of walking paths, friendly ducks, a playground and a fishin' hole where your daughters can fish for fun. POR has a small playground. POR has the boat to Downtown Disney - a definite plus and a fun trip! AS resorts share the same bus mostly. If there are lots of folks getting on at ASSp then you will stand on the bus. POR is a little more centrally located. ASMu is close to AK, but a fair "ride" to MK. For your family, if all 3 girls are staying with you, you will get a bayou room which sleeps 5. The bayou buildings are small and intimate, ASMu buildings are huge and very motel like. POR's mansion and bayou rooms have character. ASMu tends to be "full" of kids. We found many teens running up and down the halls at all hours. POR is quiet and peaceful.
One caveat, regardless of which resort you pick, you will have a magical stay! Enjoy!!