The AAA engine only came in 12v format. OBD1 cars had a distributor OBD2 vr's had a coil pack. The 24v vr6's are either the AUE or AQP engine, as found in the mk4 4 motion.
The 24v engine has a variable length intake manifold which gives more torque low down, this is a bit of a weakness with the AAA engine, unless you spend £££ on a Schrick intake manifold.
Just to add, the 12v vr isn't a true DOHC engine in my opinion. Yes there are 2 sets of cams, but neither independently control intake or exhaust valves, they just control a bank of 3 cylinders each. I think VW put the 'DOHC' logo on was because so many other cars of the era were doing so.
Take a look on Wikipedia for the vr6 engine, there is some excellent articles on explaining how the engine works and what differences there are in later adaptions.
