As far as i know SG crank is a longer one and the abbreviation 'SG' just represents the size of the crank.. An SG type crank is used in almost all touring cars... as its a bit longer.....

The other threaded type is usually used in trucks etc...

I dont think it has anything to do with performace... correct me if i am wrong ppl......

The SG crank is less likely to be unstable at higher RPM, The reason is the clutch & bell are attatched to the crank directly. On a threaded crank the clutch is attatched to a pilot shaft (long flywheel nut) being 2 peices give more chance of vibration etc.

An SG shaft allows the use of a 2-speed clutch without having to use a pilot shaft/nut which, as previously stated, is not the best for several reasons. I do not believe the SG shaft makes more power but I do think most companies use an SG shaft in their best engines.