SATA 3.0 GBps is the SATA II standard. SATA 6.0 GBps is SATA III. http://en.wikipedia.org/wiki/Serial_ATA#SATA_revision_3.0_.28SATA_6_Gbit.2Fs.29
SATA-300 is not a recognized standard and is sales-speak of some vendor.
There is no such thing as SATA II, or SATA 1 or SATA 3.
Those are Sales talk.
There is only SATA, and then you got the quality(speeds) Where you got 150, 300 and 600.
SATA 150 = 1,5 Gb/s
SATA 300 = 3,0 Gb/s
SATA 600 = 6,0 Gb/s
But the therms are so often missused it has allmost become the norm.
Check out this video from Linus Tech Tips that tells the whole story.http://www.youtube.com/watch?v=TvZr8Pr3_vE
Watch from 3:35