The most important difference is, at least for me, the quality of the content.
I’ll I didn’t play FM8, but what I saw and read about it, is that there’s still cars in the game which have very old and therefore inaccurate car models. And seeing comparisons in videos, even the new car models are miles behind the ones from GT7, the details (for example the lights, the interiors,…), the paint and the overall presence of the cars (i don’t know how to articulate it in a better way), you just feel that PD puts in the extra amount of work to reach this level that is almost divine imo. Sure GT7 car models still have flaws like low poly parts, for example round shapes, but that’s a technical limitation that I think can easily be solved with better hardware, nothing that needs them to create a totally new car model.
As much there is to criticize about GT7, as much as it lacks content in some departments, it doesn’t lack soul.