Try and get the AEX, especialy if you have had one in the past, as you wont like going back to the older type!
Basically the AEX has multi-point injection system, although still as slow as the earlier engine, it deffinatly has more about it.
The earlier engines are single point (Basically an electronicly controlled Carburetta). In my experiance these are very sluggish.
As for gearboxes, i had five speed on both types, so not sure on that.
Other then how the fuel is injected the 2 engines are pretty much the same.