I started with 5.0 then 5.1 > 98 > 2002 Pro > FSX > 2020, & soon 2024.
I can't remember the difference between 5.0 & 5.1, but apparently, they were different enough that I have both on my list of flown sims.
Found this on MSFS 5.1 right after I posted:
"In reality it is a beefed-up version of FS 5.0, with more and bigger sceneries in a 32 times enhanced resolution and much better textures. A cache system was included for faster loading. Better coastlines, night effects, clouds, textured and especially the addition of haze made the FS-world again look more realistic. More that 350 airports were now included and worldwide airport refuelling made round the world trips feasible. Finally, a performance switch was added, giving the user a trade-off between higher frame rates or better looks."
Forest