They both have slight advantages/disadvantages over each other, that work around how they implemented certain design choices making it a tough call. I feel GTS had an edge on visual prowess in some areas and falls back on others, especially when you see the weather and TOD changes that happen in Forza. However, GTS gets a significant nudge in lighting and materials, they've always excelled over the competition in that regard.
When it comes down to it, both very much Excel in visuals, but in different areas. I feel it's too close to call, as they're both visually stunning.
Edit: I should note that I'm strictly talking about gameplay situations. When introducing scapes into the mix, those photomode situations are currently untouchable in terms of visual prowess.