The 2 speeds are all the same between the T, R, RR just the clutch is different.
The T and the S are almost the same car. The major difference between them (cost to upgrade) is the 2 speed and the engine. The GS15R that comes with the "S" has always ran good for me although I have herd people say they shatter pistons.
As for the GXR15 that comes with the "T", I haven't ran mine yet.
If you want to run in a "box stock class" They are both good cars. Here they are allowing you to add a 2 speed because most of the competition has them. That makes the "T" a better buy. Now I have to see how the engine holds up.