SATA3, but would still go with the Crucial C300 128GB. Fitted another one yesterday, and they rock. When you eventually go SATA3 then you already have a built in upgrade path and they are still 'quick' on SATA2.
If money is no problem, then I would recommend a Revodrive 120. They are the fastest SSD on the planet for the money, and now I cannot live with anything slower as a result. There is a lot of conjecture about Revodrives, but mine has been faultless and still amazes me when I bench it. At 500MB/s READ and 440MB/s WRITE, there is not a lot at this price point to compete with it, plus theres no wires or SATA ports to be used up as long as you have a spare PCIE lane.