I don't believe there is a difference between the two, just small ones. The XPS has the silver/gray lining around the keyboard whereas the 9100 has the blue lining. The XPS offers the 3.4Ghz Prescott Processor with Hyperthreading (as well as the Extreme Edition), the 9100 does not. The XPS offers 3 different Quicksnap covers. All other hardware is pretty much the same. The XPS also has a few different options. XPS owners have a dedicated service/support phone line, 9100 owners have to go through the regular Dell Support. XPS owners also receive the XPS backpack for free.