The Agility 2 uses the new SandForce SF-1200 controller, as does the top of the line Vertex 2, but the Vertex 2 gets a special version of the firmware that gives the Vertex 2 higher random write speeds.
But otherwise the two drives (Agility 2 and Vertex 2) are the same. In real world tests the difference isn't that significant, not $120 significant anyway unless you are doing something very specific where the random writes will make big differences in your productivity.
The Vertex 2 is probably the best SSD out there right now, and this gets you 95% of the way there at a very good price.