I couldn't get my hands on a 500GB AHCI M.2 blade, so went for a 500GB Samsung 960 EVO NVMe. I have my boot OS on a 500GB Samsung 850 EVO sitting in an Accelsior S SATA III PCIe card, and wanted a blade to Fusion with my 2TB WD Black for my data so didn't need the blade to be bootable.
As for speed, the Samsung 951 AHCI is slower than the Samsung 960 NVMe - check this
link for a speediest comparison. The 960 EVO is capable of 3200R/1900W
BUT, what I didn't realise is that my cMP 4,1>5,1 PCIe speed is limited to 2GB/s, so the best I get is around 1500RW. Check out
this post and
h9826790's explanation about speeds.