In the m.2 form factor for SSD’s you get the SATA interface and PCIE interface, the PCIE version being much faster.
Take stock of how many programs you use and what their requirements are (libraries, etc.), also remember that SSD’s wear out quicker the more writes there is to the memory modules, thus larger capacity SSD’s will last longer than small capacity SSD’s (based on the same amount of data on them with similar amount of reads / writes per cycle).
Typically a drive for storage needs to be higher capacity, but the SSD versions (either PCIE or SATA) of these drives are more expensive than mechanical drives.
My personal machine has a 512GB m.2 PCIE SSD of OS and apps and for storage I have a 1TB SATA SSD. I also have external mechanical HDD’s for backups and archives.