Maybe it will sound biased, but it's based on expirience.
If MSFS flight simulator is the only "game" you play then staying away from ATI and having a nice relationship with Nvidia means staying out of trouble.
I'm dealing quite a lot with building systems specifically devoted to nothing else but running MSFS for last seven years and this is my true, genuine expirience. I have possibility to borrow almost any hardware there is for a period of 7-10 days, so I'm in position to check almost any combination of hardware thoroughlly. My latest comparision was made 2 months ago with certain range of ATI cards against Nvidia's and results, even if expected, were still even more off than expected and were definitely in favour of Nvidia. With this statement I'm not trying to say, that ATI cards are bad piece of hardware, I even have information from some of my customers, that some other 3D games run better on ATI cards than on Nvidias, but that definitely is not the case with the MSFS, especially so with FS X. I never managed to discover why that is so, but paractice and real-life expirience shows, that is so. Maybe with tweaking of drivers on some expert level the situation of ATI against Nvidia could change, but that is something I'm not aware of at the moment.