1. Vertex 2 drives are better than Agility. They have 5x more IOPS then Agility drives (50,000 vs. 10,000)
2. Pay close attention to the physical size of the drive. It will be either 2.5 inches or 3.5 inches. The bigger drives (3.5 inches) are less money usually.
According to the Passmark 7.0 benchmark, users of the 3.5 inch Vertex 2/Agility 2 have received slightly faster boot times and benchmark scores then the 2.5 inch drives.
http://www.harddrivebenchmark.net/high_end_drives.html
I have no idea why the 3.5 inch Agility II ranks best on that chart though. It shouldn't be lol. For SSDs, more IOPS = better.
Make sure you enable AHCI in your BIOS though or you will NOT get the speed you're paying for, more like 1/3.
I'm just re-emphasizing what blackhawk1928 has already said with numbers and details.
After speaking with an OCZ technician, the higher capacity the SSD for the Vertex/Agiility series, the faster (by a small amount) they are. This is only true up to about 240GB though.
As an owner of (2) Vertex 2 60GB drives (2.5 inch), one of which is used to boot, you will not be disappointed, so long as you're in Windows 7 to utilize TRIM.
My boot time with an old school SATA II 7200 RPM hard drive was about 90 seconds. With the Vertex 2, I have a working desktop in 25 seconds from pressing the power button on my computer.