I made a review of the FSimStudios version, in which I compare it as best I can with the M’M and FSDG competition (bear in mind I don’t own the other two). FSimStudios’ is the best airport but FSDG’s offers a more developed city. M’M’s doesn’t have a city, per se, but does feature numerous landmarks.
Overall, I concluded that the best product overall is FSinmStudios, and I’ve not been disappointed.