the speed to load games, recently i had this situation with a hard disk and a ssd
for space limitation with the ssd being only 120 gbs, i moved gta v from the ssd to a hard disk, to my surprise the speed loss was very small, a couple seconds more loading some things
if you have a decent ssd(mine is sata kingston) and a decent hard disk, for gaming you don't really need a m.2 ssd unless you want less space ocupied on sata, less sata cables and a nicer pc it would be a good reason to buy one and use the m.2 sata speed slot
they are about same price than the sata ssd, you don't have to have them but is nice to have them to boot and for multitasking
what is very useful is have a ssd being sata or m.2 to load windows, that is really nice