I have both an XPS and MBAir (2012 model).

I routinely boot the same linux-USB desktop distro on both.

The XPS is probably a bit faster (i7), and screen DPI is better. But, MBA provides and overall better experience.

The most glaring things that stick out in MBA favor is: (a) keyboard, (b) touchpad.

The XPS touchpad is really sub-par given the focus of the machine as a development platform. Often while typing, the slightest brush against it will cause the cursor to jump. This is not an issue on the MBA.