Disregarding for the moment subjective issues like design and operating system, I would ask two important questions. How is the battery life, and how is the build quality? That's usually where Windows PCs quickly lose any desirability (to me) compared to Macs.
Also, I notice that this XPS is 0.6-0.8 pounds heavier than the new MacBook, and as far as I can tell the XPS has a fan.