I can't speak for MPA3 since I never looked at it, but anyone considering using it has to weigh in the fact that many webmasters will stay away from it.
NATS on the other hand is feature packed, has great support, and the NATS team keeps adding new little features weekly that make our lives easier.
There may or may not be any features in any of the softwares that are completely exclusive to them and are a must, but NATS has good reputation, good feedback and good support. Can't go wrong
