the 6gb/s refers to the interface speed rather than the actual speed of the drive.
I used to have a crucial M4 mSata SSD which I thought was good at the time, real difference from a mechanical drive but now I have a cheap sandisk sata SSD which is even better.
The mSata SSD was directly on the motherboard which would be better than a PCI-mSata card anyway so by using the card you won't get the SSD's full potential.
The whole process of converting one interface to another will slow things down and the PCI bus isn't a fast bus to begin with so you're better off with a sata SSD.