The record and playback amps might use essentially the same circuits, but that is about as far as the similarities goes.
The RT-909 has a much more sophisticated control system, that I have been led to believe may even include electronic braking. It also uses a Belt-drive system for the heads, so the motor controllers are probably different too. Also, the 909 needs to have additional support cuircuitry to drive the florescent meters, and the tape counter which the 707 lacks.
Both are breathtakingly gorgeous looking machines, but personally I'm just not a fan of their sound.