The only difference worth noting is that one will work and one will not. Beyond that, it's your call.

dont

Just because you can make everything fit, doesn't mean that it will work properly. Without previous experience, you'll not know where to find the problems. Much better to buy a complete AEG like what everyone else is saying.

If you insist on building it yourself, spring for a ready build Systema gearbox and build around it.

But a ready built gun is your best choice. least amount of pain and frustration.

If you dont know what a v1,2or 3 gearbox is... You have no business trying to make a gun