XPS 8900 add SSD Sata or M.2

I recently purchased a 8900 and am looking to add an SSD and possibly upgrade the VC to a 1060 or 1070.

My question is about the SSD, I am leaning towards a 250gb evo 850 but am unsure of what form factor will be best. I understand that the m.2 is restricted in the lanes and so have read that I should use a pcie slot instread. I am confused as to why not just a SATa

Thank you for the help