Finding a wireless card that just worked in Linux is still a big pain. Wireless support in Ubuntu has gotten better but and still needs better documentation. Ubuntu's own site has outdated information that is badly in need of a maintainer and consistent updates. It doesn't have the Dell XPS13 listed, even though it is a Ubuntu developer machine that ships with Ubuntu preinstalled that came out over two years ago.
You would think with its popularity there would be Ubuntu certified devices, and not just recommended ones. Think Penguin has numerous Linux certified cards in stock and I would highly recommend buying through them if you can afford to pay a little more. I was strapped for cash and was willing to do a little leg work.
I ended up going with the TP-Link TL-WDN4800 N900 Wireless Dual Band PCI Express Adapter.
The TL-WDN4880 has an Atheros (ath9k) chipset inside. Making it plug and play with modern Linux distributions. In Ubuntu 14.04 you don't have to install any drivers or configure any settings. I'm very satisfied with my purchase and not regretting going wireless.