Standard 2.5″ slot supports SATA interface, disks are mostly cheap and available in larger capacities. For Librem laptops they require aluminum frame which holds them in place. Maximum disk height that will fit in Librem is 10mm.
M.2 supports SATA and PCIe interfaces. Both SATA and PCIe M.2 are very similar from the outside: they are smaller, compact and easier to install, compared to 2.5″ disks. To install one in a Librem laptop you only need one screw. Librem laptops support B+M and M keyed 2280 sized disks (the sockets themselves are M keyed).
SATA M.2 disks do not offer speed improvements over 2.5″ SATA SSD disks.
PCIe M.2 disks using NVMe interface offer quite the speed improvement compared to SATA M.2 or standard 2.5″ disk.
For M.2 Purism offers:
* SATA disks (sequential read/write speeds: up to 550/520 MB/s)
* NVMe disks (sequential read/write speeds: max 3,200/1,900 MB/s)
* NVMe PRO disks (sequential read/write speeds: max 3,500/3,300 MB/s)