I have made lots of comparison on different tracks with different weather and lighting. I will post them later today but first i will say this. Both games look good, and soo much better with HDR. Played FM on pc and GT7 on PS5 on my 55inch 4K120hz OLED.
Forza has superior environments but not always. Things like grass/bushes and some textures look better in GT7. But Most of the track detail is much much better in Forza which shows the benefits of them rebuilding and rescanning the tracks for current gen. The tracks are filled with POM in Forza which look really good. Trees are also much much better in Forza. However there are times that with the right angle and lighting (when the sun shines on the tree textures) GT7 trees can look really good. But if the light is coming from behind the tree they look bad and very dark (almost black).
Lighting is literally mixed, sometimes GT7 looks better, sometimes Forza looks better. Forza has better atmospheric/volumetric lighting while GT7 nails the colors most of the time. The RTAO is really good in Forza, its helps with the interiors and tracks. GT7 is also not bad, not Forza good but they did a decent job without using RT. GT7 uses RTAO in photomode and you notice the difference immediately.
Things like in game car models are again higher quality in Forza because of the use of photomode lod which is not the case in GT7. GT7 still uses lower lod during gameplay in all modes. Interiors are closer this time but i will give slight edge to GT7 because the older cars still have better interiors most of the time. Newer cars are pretty close and most differences are the materials and shaders. Shaders are massively improved in FM and they look amazing but so do they also in GT7.
The overall reflections are much much better in FM (not perfect tho). GT7 has lower quality cube maps and some low fps reflections in the cockpit. Also the RT really adds depth to the cars in FM, once you get used to it it looks off without it. But he RT has also its own problems which i hope they can resolve.
IQ is much much better in GT7, cleaner and more stable. Forza has some pretty bad iq issues which they need to fix.
Draw distance is slightly better in FM.
Shadows are mixed, sometimes the track shadows in Forza are low res and sometimes GT7 shadows are low res. It all changes with different lighting and tracks.
The weather effects in Forza are much much better but the lighting is too bright in daytime rain. GT7 looks more accurate but the effects are not there to back it up.
The crowds and race atmosphere is excellent in both, crowd look really good in both games.
I played and compared both games in Race mode so the tracks are filled with people and objects to make everything fair.