The primary difference between the SR9 and the HK91 is the barrel. The SR9 has a heavier 20" polygon barrel (as opposed to an 18", relatively light, conventionally rifled barrel).
The SR9T is the SR9 with the MSG90 buttstock, PSG1 trigger, and PSG1 grip.
The SR9TC is the SR9 with the PSG1 buttstock, PSG1 trigger, and PSG1 grip.
The SR9s are very fine shooters, but I've seen both examples shoot under MOA with their preferred ammo. The primary advantage of the SR9 is that it is easier to clean, and the T and TC variants have FAR superior triggers, HK triggers normally being horrid from the factory.