Dont look at SATA II/III as one of the factors in deciding which SSD to get. You will hardly ever see any speeds that SATA II cant handle. As XSCounter said, the OS uses mostly 4k range files, and those dont come anywhere close to the advertised speeds (which are usually sequential read/writes so that they can show you a big shiny number for you to buy it). Dont go by the advertised speeds. Go look up some reviews on them on anandtech or thessdreview.
The C300 is actually not more expensive just because it is SATA III. They were one of, if not the first, to jump to SATA III so the C300 is actually a generation older than the SATA III models out from other companies now. Its very good for its price.
And there was a huge thing about OCZ switching the Vertex 2 from 34nm to 25nm without any sort of warning to anyone and the drives got a performance hit for the 25nm versions. They had no separate label or UPC or anything so if you bought one you had no idea what you would get until you got it. Now they have 2 different product codes for them. They are the same except the 25nm versions have an E in the code (like that Vertex 2 drive you linked). The 25nm are now priced lower than the 34nm versions, so if you dont mind the performance that theyre at (theyre a bit slower than the 34nm, which is what the outrage was about, but theyre still pretty damn fast) they may be an option still. Or you could avoid the whole headache and avoid it
Note that 25nm drives are not inherently slower just because they use 25nm NAND, it was just the case with this drive going from 34nm to 25nm because of the way it works.