If you don't want to watch, this is the summary.
GT Sport wins by a hair.
They both are amazing looking games. With that said PD has always had a superior lighting engine that seems to manifest itself in every one of their games.
Either way my impression from this is that you can't go wrong with either game, but content wise Forza has the edge. Consensus seems to be that GT Sport is the better looking game, but Forza wins on content.