Short answer.... speed is not that important.
Longer explanation:
Small random I/O at a queue depth of 1 or 2 is what we do mostly.
And for that type of activity, performance is remarkably comparable for both sata and pcie devices; regardless of m.2 or 2.5" devices.
These guys could not tell the difference:
View: https://www.youtube.com/watch?v=4DKLA7w9eeA
What you see advertised are benchmarks with queue depths of 30 or so and focusing on sequential performance.
The sequential performance is impressive and the pcie versions are 4x better or more than a simple sata 2.5" drive.
When attached via pcie 3/4/5 sequential performance goes up impressively.
The reality, though, is that you will not notice such performance except perhaps for a virus scan.
The value of m.2 devices is the smallness and ease of attachment.
The good news is that the cost of a device does not seem to depend on pcie vs. sata.