Hi Mark,
Those two are virtually identical.
Things to consider:
1) QHD+ doesn't work in Chrome
2) You won't have any use for the touch screen
3) The M800 comes with two 500G drives (SATA and SSD)
4) The M800 is available with a lot of options and customizations that the
XPS15 does not provide.
5) If you need professional 3D graphics requiring the Quardro chip, get the M3800.
I am wondering why you are not considering a system without the high end graphics (for AutoCad or Architechs/Gamers) and without the touchscreen or something from Alienware (17 is 1/2 the price with optional overclocking and LONG battery life).
These choices are comparable to a Mac PowerBook in price!
References: