After putting time into both games it's pretty much this.
GT has amazing non-gameplay (although..it's still technically the game..so that term feels weird) moments visually. Scapes, good DOF on replays putting the focus on the cars, the way the cars are lit, the general lighting of some tracks. Other cars look nice during a race. At certain time settings the game can look really good in motion (see low sun, long shadow, high contrast moments).
Forza 7 has the better graphics during actual gameplay across the board in my experience. Textures are much nicer/cleaner during a race, LOD is better, IQ is better, the environments as a whole look much better during an actual race (especially on a 4k screen). Forza 7's replays suck (why can't Turn 10 get this right!!...I think it's worse than 5 and 6). Maple Valley, trees wise is disappointing....really disappointing.
Forza 7 seems to use DOF or something during races too. The way fog/smoke/haze fades into the distance creates a real...Idk....depth, I guess is the right word. GT in what I played..everything is just sort of "there" visually. Things are janky looking in the distance, textures are really blurry more than 20 yards from the car, some backgrounds are too low-res to be on a big screen (the screenshots look nice on small screens, especially a phone). The trees in GT, while I find the way they are lit to be nicer, they're often a mess anywhere more than 50 yards away visually.
The skies in Forza 7 are a different beast altogether. They're dreary at times (purposefully), they're dark and menacing, they're absolutely breathtaking at times (thanks Horizon 3). Are GT's static?...they all looked static while I was playing. The backgrounds (like distant mountains) often looked like static (not 3D) textures when in the distance too.
Anyway, I think both games are worth buying for their own unique things and they both seem to be really good games.
This is a graphics thread, so for me I guess this is my general thought right now:
In-race graphics (on an actual TV/Big Screen) overall = Forza 7 Wins
Car details/Photomode/Scapes/Car Materials/Replays = GT:S Wins
Neither one is the hands down, owns everything, winner...not close on either side.
Thank you telling us which plateforms are yours my friend...
Do huh?