Remember that m.2 is just the form factor. The PCI-E variant is noticeably faster than SATA, but whether it is enough to justify the cost is subjective.

SATA SSDs in striped RAID can be awesomely fast, too. In theory up to "6X" faster, although in practice only up to maybe 2GB/s throughput with six fast SSDs.

There are some PCIe SSD cards which can sustain 2GB/s, 4GB/s, or even more throughput. The G.Skill Phoenix Blade is a fine example, actually a "low end" enterprise product rebranded for gamers.

SATA, M.2, and PCIe SSD are just form factors, they all use basically the same flash chips and controllers. Performance at the high end is limited by the bus, and PCIe is the fastest of these. Unlike SATA and M.2, PCIe cards are often multiple "discrete" SSDs embedded onto a single performance RAIN/RAID configuration, reported to the system as a "single" drive.

I hate how they merge multiple techs into the same form factor. M.2 has sata, x2, x4 and also nvme memory (not supported?) it makes devices not consumer friendly. In addition to the cost I just purchased a 850 evo 512GB and will run windows/apps from that. I may upgrade to an x4 in the future when costs come down a touch. Thanks for the info guys.