I believe you have:
6x SATA ports.
1x M.2 slot (PCI-E 3.0).
M.2 drives are usually cheaper and more plentiful. A lot of the old SATA drives have also been downgraded since they were first released.
For general usage (e.g. desktop, storage), it doesn't really matter which one you have.
For large file copies and for loading lots of photos, or videos, I'd get the M.2 drive.
For games, they're starting to stream assets and use DirectStorage more than before, so if you're planning to play newer games then I'd go M.2. For older games, it is not important either way.
Make sure you have the screw and standoff to attach the M.2 drive to the slot (assuming it uses a screw, most do). It might be in the box (e.g. inside the manual), or already attached to the slot.
