Neither. The stop and go advanatage has to do with the fact that at slower speeds the engine doesn't need to spin over hardly at all, whereas it must run at idle most of the time in stop and go traffic in the case of the Honda. The Prius tranmission is basically a single fixed-gear ratio, which uses a differential to split engine power between the wheels, and a generator. When it needs more torque in lower gears, more of the power is diverted toward the generator, which then spins a second high-torque electric motor on the driven end of the transmission.

The drawback of the Toyota system is that at highway speeds, this method must be run in reverse, to "gear up" the car, meaning that the system continually wastes some energy at highway speeds by converting mechanical power to electrical power, then back.

The Honda system however uses normal geared (or belt-and-pulley) transmissions, so it wastes less engine power at highway speeds in the transmission. However, it wastes more at slower speeds.

The GM system sounds excellent on paper from an engineering perspective, and should feature the best of both worlds (between the Honda and Toyota Systems) whenever it's actually released. It has the power split feature of the Toyota system, which should make for good city mielage, but it can actually shift into a true high gear for highway cruising, meaning it should drastically cut down on elecrtic conversion losses. This will be combined with other fuel-saving systems such as cylinder reduction.