There are many tradeoffs made in both games, and their respective developers went with what they thought was best.
IMHO, GT7 cars, roads, grass/shrubbery, and general attention to detail are better overall, but FM has better anti-aliasing (at least in some of the scenes that John chose) and greater populous aspects such as tree density or certain crowd scenes. FM has the more technical RTGI/Shadowing showcase; however, the techniques look more subdued by other tradeoffs, such as the flat and generic-looking grass or the paint (shaders) quality and reflections looking somewhat drab (or overly saturated with light) because of the RTGI/Shadowing. Buildings, houses, and overall course designs are pretty much a draw, but certain structures in FM look better, IQ-wise.
Anyhow, GT7 still blows to me, regardless of its next-generation status. When stacked against Forza Horizon 5 (yeah, I know, one is more of a simulator and the other is more of an arcade racer), GT7 just looks dated in comparison, both artistically and tech-wise.