Yes they are both version 3 gearboxes but the nozzles and selector plates are proprietary. The rest is pretty standard. Hop up units are proprietary so thats limited.They use medium type motors there are quite a few out there but not as many as if it was long or short type.
External furniture is limited so not much you can change there. Mag selection is also very limited.
I have had the bolt cover on my Ics sig break twice because its made out of crappy plastic and it the only one you can buy, I am making a new one out of aluminium but thats had to be wire eroded and machined. Also I want a metal hop up unit and I can only do that by making it myself. Not everybody is lucky enough to have access to the sort of equipment needed to do things like this.
If you buy an M4, AK or MP5 you can replace or upgrade every aspect of it quite easily.
Not knocking either Sig I own both and the Ics sig 552 is my primary, the G&G I brought used and it needs some tlc and will be my next project.