I tried both, I had the Gamepass too during months before losing my hair too much with MS Store and deciding to buy from Steam.
Try to know where a game is installed with the MS Store version if you don’t remember where you install it when it asked. Impossible, the GUI never tell you where your games are.
Tried to uninstall a game (The medium), MS Store installer said it was removed, none of my disk get their free space back. I had to change right of the WindowsApp of all my 5 disks to get access and finally find the game. Deleting it wasn’t permitted (I’m Administrator). I tried all. I had to remove all my other stuff from the disk and format it low level.
MS Store often proposed me to reinstall games I didn’t know they exist and never bought. And yes, changing the MSFS executable properties to not use “Fullscreen optimisation” which create stutters is not permitted, or adding a simple redshade. You don’t have the control on your computer.
So I switched to Steam. I have access to all directories, and I’m happy. No problem to install any update, like we can read each time an update of MSFS land for several MS Store users, looping on a file, GameService not updated, people having to dig in registry to fix MS Store, etc. all that don’t impact me anymore.
I’m an happy owner, and full Master of my machine.
Saying all that, if you bought stuffs and addons for MSFS through the MS Store they will not be transferred on the Steam installation…