This is ridiculous, there's no way or shape to compare both games in the graphical department. The sheer amount of assets that Forza 5 can render thanks to it being a much higher end hardware can't be matched by the assets that GT6 can render. It's only natural, we're talking about two different generations of hardware here.
These kind of comparisons are really getting out of hand now, and it's only a breeding ground for a useless and pointlessly long amount of banter from both sides that will only fuel the senseless "console wars".
If we're talking about exclusively about games on the same hardware, from what I've seen then I'd give the edge over to Forza 5, mostly due to the lightning engine, which I found to be just lovely.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
