First off i am not trying to start a fan boy war or to say one is better than the other they are two different aproache's to a driving based game.
Having played both games from early versions there is some things in Forza that could really improve GT.
Livery's & Editor.
Manufacturer Races Remember Carvalho Ferrari challenge ?.
The Sound of the cars.
DLC packs worth buying Porsche pack for example. (Forza maybe had too many)
Dev's racing and talking to the community and rewarding players with rare cars and even DLC for beating them or just driving with them and for even watching one of them create a custom livery.
Having played both games from early versions there is some things in Forza that could really improve GT.
Livery's & Editor.
Manufacturer Races Remember Carvalho Ferrari challenge ?.
The Sound of the cars.
DLC packs worth buying Porsche pack for example. (Forza maybe had too many)
Dev's racing and talking to the community and rewarding players with rare cars and even DLC for beating them or just driving with them and for even watching one of them create a custom livery.