Just remember NVMe won't give you much improvement in most applications asides maybe large data Transfers. In most cases, when the SATA is cheaper, the SATA is often best value for money. NVME is often only faster in artificial benchmarks.
OP as Zippy has stated, M2 is just the form factor, the interface also has to be supported (SATA/PCIE)
Much better explanation than mine haha. Yes SATA is definitely better value and unless your working with large files or require demanding read/write speeds; a SATA M.2 is much better value for the money.
I just want an m.2 for my OS and a few games as I will be attatching a bunch of mechanical drives to it and I can get a 500gb for £58. I use my mechanicals for recording game play footage.
SATA it shall be when I can afford one and 2 ssds also as my current case has 2 trays for mechanicals and 2 mounting slots for ssds or laptop drives.An M.2 SATA is all you really should need, if you will be working with large files then go for the NVMe