I have both axles in front of me right now.
The Proline plastic is smaller flange to flange, it measures .395".
The AE plastic is larger flange to flange, it measures .472, the extra material making the hub wider.
The Proline axle is longer, 1.280".
The AE axle is shorter, 1.180". From the back flange to the roll pin is about .050 smaller too.
The AE axle has an c clip behind the outside bearing. When you tighten the wheel, the 12mm hub clamps the bearing between itself and the C clip, turning the inner bearing race into a crush sleeve. The Proline kit has nothing in there to keep the bearing from crushing itself, except for the outer hub making contact with the roll pin. One might be inclined to space the out of the Proline kit to make the front end wider by adding shims between the outer bearing and the 12mm hub. Take my advice (for what it's worth):
Don't. It won't turn free and it will destroy the bearings in a single race day.
My observation is the weakness on all these 12mm conversions is the lack of a crush spacer. The hinge pin runs right through the middle of the axle (that's what the relief in the middle of the axle is for) so there's no room for it.
You can chuck up a Jconcepts wheel on an AE axle with a Jconcepts hex installed. Retaining the roll pin will becomes the problem, because it's no longer captured by the hex. You could CA them in, it's might not be pretty, but it will work.
Here is a picture. The longer one on the left is the Proline unit:
