I found this tech article on another site and liked it a lot. The information has been supplied by MMR. Enjoy

The Jessel Aluminum is on the left, dual roller design for decreased friction, the higher the lift the more power to be gained, this is by far the lightest available and was a very limited production, we used a set of these on the worlds fastest 2V with great results.

Next is the Jessel Steel rocker, dual roller design for decreased friction, the higher the lift the more power to be gained, same features as the aluminum but heavier, this is the rocker we now use on most of our race applications (2v,3v and 4v)

Next is the GT500 / Ford GT, single roller design, this rocker was basically turned inside out for extra cylinder head to rocker clearance, some people think they are a better, the only downside we have found is because the pin on the roller intersects less of the rocker, we have had the pins come out under high spring loads. The remainder of the body is actually a little more rigid than the other stockers which is helpful.

Next is the FR500 rocker, single roller design, basically a stock rocker with additional clearance for a longer valve stem/ spring retainer, it can be seen in the bottom picture where the valve sits in the rocker groove. This rocker is slightly lighter than stock but has no other benefits.
On the end is the factory Rocker, single roller design, a great part for the money, they have good long life