It's actually a little more complicated than your question...
M.2 is a form factor (like a specific size/shape). You can get "SATA" M.2 SSDs which use the SATA 3 interface and perform identically to the 2.5" drive variant (that's the "regular" SSDs you were thinking of) of the same SSDs. So, for example, popular drives like the Crucial MX300 and Samsung 850 EVO come in both "regular" 2.5" form factors, and in M.2 drives which plug straight into most M.2 slots on motherboards. They perform identically, with the M.2 drives saving space and avoid cable clutter, while also taking up an M.2 slot which you might decide you want to use later for a faster M.2 drive (many motherboards still only have 1 m.2 slot).
Different M.2 SSDs, on the other hand, can use the NVMe protocol and communicate with the PC over PCIe express lanes (just like a discrete graphics card). They're usually faster (theoretically) and more expensive.
I'm guessing you're asking whether you want an NVMe SSD, or a standard SATA drive? For the vast majority of people in the vast majority of use-cases a SATA SSD is all you need. And most people would be better investing their limited money elsewhere, or getting a larger SATA SSD which can fit additional data on it. If you look at real world testing like Windows booting and application loads, most SSDs are barely distinguishable from one another.
Unless you have a large budget and are running out of useful things to spend your money on, just get a SATA SSD. You can still get an M.2 SATA SSD if you want to avoid the cable clutter.
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