Played both over the weekend, GT's online racing is incredibly well done. I'm honestly shocked at how smooth it was and the precautions they take. The ghosting feature needs to be copied by everyone asap. It's more like a polished Project Cars than any other previous GT or FM.
Forza gives you more game for the money at this point in time. Though they do some things wrong in this version VS 6 (loot boxes, physics going ape when brakes are applied, presentation imo is way down and not as good) but again, you get more game. I dislike the car tiering and career progression but that's what you gotta live with for now.
I hope they take GTS's gameplay and just start creating car models for a GT7. If it was its own mode instead of a full fledged game it could easily be the best in the series. It's that damn good... As an online race mode. Not as something to sell the game on. It reminds me of Titanfall 1. Excellent execution but no meat on the bone.