I can only answer as a user of nats that I'm pleased with their product and service and do recommend them. It seems that the guys at Nats are continously working on new features to improve the product. I can't talk for mpa3, those that use it seem to like it as well.
There will be people who find faults or sing praise for both, it all depends on which one you feel more comfortable with and which fits your budget better.
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