Given the same specs, the xps15 and m380 look exactly the same and I can't figure out why marketing is trying to sell me on the $500 markup for the m3800. The only variance is the video card m380:k1100m and xps15:750m and the 750m seems to be better!
m3800 is "workstation-class" laptop, so its graphic cards is optimised for CAD applications.
XPS is run-of-the-mill laptop, and if there is an add-on card, it is more for gaming / generic applications.
Sorry, must drifted somewhere on XPS. The list I've seen next doesn't refer a dedicated graphics card, but has the touch screen as well. However that 2GB NVIDIA® Quadro® K1100M GDDR5 is a graphic card for advanced professional use. This brings to what Alabalcho posted.
M3800 is a advanced work station for professional users and XPS an advanced entertainement/gaming for home users. Probably they won´t performe as good when we switch the use.