Considering my first VR. I would only use it mostly for MSFS2020.  Is 128 adequate or is 512 better?

Judging from the Quest 2, 128 will suffice. MSFS is not loaded into the Quest, so the extra space won't be useful for flight simming. However, when you load additional apps, they would be saved on the Quest, and then 512 gives you space for more of them. I got the smallest Quest 2 option and it was perfectly adequate for my purpose, which is flight simming plus a few extra games.

Thanks.

I use Quest3 for PC stream only and 128mb would probably enough for me...