The difference between the plastic body oil filled shocks and the aluminum body oil filled shocks is the body, the piece that is inside the spring. The aluminum body shocks are tougher and are less prone to leaking.
Now, the differences between the VX .18 and the Infinity .18 are:
1) The Infinity makes more power than the VX, however the vertex will actually end up lasting longer.
2) The Infinity doesn't have an adjustable low speed needle, where the VX does.
Those are the two main differences between VX .18 and the Infinity .18.
There is no difference between the diffs in any of the tornados. The reason it is called the two bevel differential is because it has two beveled gears inside the differential gear set.