GT7 is punchier/more contrasty like it was filmed through the lens of an earlier Samsung Galaxy camera, whereas FM is more flat like an earlier iPhone camera (though it's funny how this seems to have switched in recent years but I digress).
I personally won't make a judgement until I'm sitting down, playing Forza, and seeing it with my own eyes in a native and uncompressed resolution, but both games look great to me here. One is punchier, and more people like punchy colors, where the other is more subtle or subdued, and it looks like the time of day greatly affects that also. It depends on what style or flavor you like, that's really it.
Hell, Assetto Corsa: Competizione is far more hardcore and sim like than either GT7 or Forza, and its image is pretty subdued, even with all the fixtures cranked up. It's still a very pretty game, though, and Forza looks to be no different, and it has RT/RTGI in-game.