G'day Silverhunter,
The actual generators should be or can be identical, the difference is that whereas the APU and it's generator runs at a constant speed the engines don't and some means has to there to be able to run the engine generators at a constant speed while the engine speed varies. This is necessary to keep the frequency of the A.C. power supply, from the engine driven generators, constant as the engine speed varies. As well as keeping the frequency constant the generators on a multi engined aeroplane are also synchronised when the various outputs are fed into a common electrical bus. The frequency stability and synchronisation are all handled by the integrated drive part of the IDG. Each IDG has its own variable speed drive and oil supply and controlling electronics to ensure that it all works as advertised.
Hope this helps, regards, more info if you need it,
BH.