Technically the SSD connector is an M.2 Interface.
As to what your system is using it's the PCIe version of M.2 (per Apples documentation), as such only PCIe SSD devices will work (you can't plug in just anything). In fact, Apple is still currently the only supplier for the correct SSD units to fit and work in this system.
As to speed: PCIe is the next step in I/O speed, being faster than SATA III (6.0 Gb/s) the current fastest SATA spec out there.
I would recommend getting a fusion drive model Vs a HD only model as the amount of work to get to the M.2 connector to put in a SSD later is a lot of work (if that is your direction).