xps 8900 question

m.2 is a newer shape for a ssd. it kinda looks like an old style memory stick with the edge connector at one end. Often the m.2 ssd is less expensive then the same ssd in a 2.5" shape because the manufacturer does not have to add the 2.5" case to the ssd, which are just NAND chips. The following link explains it briefly

http://rog.asus.com/308552014/labels/guides/ssd-guide-pci-express-m-2-msata-and-sata-express-the-differences-explained/

also the m.2 often will allow you to connect the faster PCIe ssd. although the XPS 8900 seems to limit the m.2 slot to 10Gbs