pci ecpress ssd isn't really practical in most systems.
you can't raid them for starters and pcie slots are already a precious commodity on most motherboards with only really the x1 slot being left over, the x16s and x8s are taken up by graphics and a lot of motherboard manufacturers like to put slots too close together, on my last board only one of the two x1 slots was available, the other being covered by a two slot graphics card.
sata III all the way.