Hi Dan,welcome to the forum. As has been said, the GT spec gives you better resale and includes leather heated seats, cruise control Bose stereo, upgraded wheels and bluetooth. If you want these then fine. If you prefer to save a little money and spend it in mods then go the non gt route.
Roadster is about the same but doesnt have the Bluetooth and is about 40kg heavier than the coupe.
Mods tend to be cosmetic as the ecu compensates and cancels any stated improvements such as pop chargers and exhausts. These, with wheels tend to be the first popular mods.
The bridgestone tyres are also not too popular generally due to a percieved lack of grip in the wet.
Thats your starter bud, enjoy the search