I've built and run both. For bullet proof quality and long life, go with the Staton. For lighter weight and simpler instalation, the GEBE's the one. Both are very good systems, and you have a choice of final gearing with both, so you can customize for speed or torque. Chains will outlast belts, but on the other hand, belts aren't all that expensive. Both are a pain in the a** to fix a rear flat tire with, though the Staton is worse. The chain drive is noisier than the belt drive, but most engines, especially 2 strokes, will drown out the drive noise anyway. I think you get better value for your dollar with Staton, and they will last forever with proper maintenence.
With either drive system, get the strongest rear wheel they offer. I speak from experiance here. Both offer very good rear wheels and the rear wheel and the brakes are two areas you really don't want to skimp on, trust me.
The Tanaka 40 will have more torque and power, if more noise. The 33 will give better gas mileage. Both are well made engines. I'll stick with my 4 cycle Robin 35s.