The difference is in the firmware...(it's a long story....but here is my undestanding):
In the beginning
there was two Sandforce chips, SF-1200 and SF-1500. The difference between the two was any drive using the SF-1500 had a large capactitor built in so in the event of a power failure, data that was already sent to the drive would be able to be written). This was an enterprise freature for servers.
The second difference was some (gets convoluted) SF-1200 chips were limited by firmware design to only write at a rate of 10,000 IOPS.
Much more better explained here:
http://www.anandtech.com/show/3661/understanding-sandforces-sf1200-sf1500-not-all-drives-are-equal
Now, since then sandfroce drives has been using the SF-1222 chip (this may be the second run of thier chips with the first run producing the SF-1200 and SF-1500 for the vertex LE drives). But I can't find where/when the SF-1222 came in and the SF-1200 went out. But from what I can gather there's no discernable difference, the difference in performance is all firmware.
*edit* the difference may also be the amount of over provisioning the controller has....I'm just guessing though.
There was a debacle over who could use what firmware and companies jumping the gun and selling non "release candidate or mass production" firmware. I have no idea the who what when where why or how companies determine which version of firmware each drive ships with now though. The cost difference between the drives is extremely small, so I always recommend getting the unrestricted firmware drives.
All that being said, here is the difference between 10k IOPS write limited firmware and the unrestricted firmware (i.e. Vertex 2 vs Agility 2)
Review of Vertex 2 and Agility 2:
http://www.anandtech.com/show/3667/oczs-agility-2-reviewed-the-first-sf1200-with-mp-firmware/2