Scenery-wise, FS is hopelessly superior. Many international airports are already in solid quality in the default versions, plus plenty of addons available. I myself have around three dozen German payware airports installed. They are so detailled that they could serve as tourist guides.
On the flight models, I strongly recommend and encourage to use external aircraft in FS. I NEVER use the default Microsoft planes, never. Usually they are superior to Microsoft'S default aircraft not only in avionics complexity, but flight model as well. Flight Simulator leaves the factory as a game, not so much as a real sim. Due to it's modular character you can (and must) tune it to become less a game and more a simulation.