Don't get caught up in the "m.2" hype. A regular 2.5" SATA III SSD will do just fine.
I agree 100%.
An M.2 drive only makes sense if you either have not enough room (i.e. in a laptop) or if you use an NVMe drive.
Connecting an M.2 drive to the SATA interface will result in the exact same performance as connecting it to a SATA3 port ... and in addition, populating an M.2 SATA slot will disable one of the "regular" SATA ports with most motherboards. No gain.