I wouldn't recommend PCIe ssd because they're just a pain. My advice would be to run 6 x 512GB SSD in RAID-10. The thing to remember about ssd drives in an array is that there's no trim support. Trim is usually not that big of a deal in a database environment because most data doesn't get deleted. Another thing is that you would need to over-provision your array to say 300GB. I would only recommend Intel or Samsung ssd brands, don't buy any other brand. My favorite are the Samsung 830s.