avoid dell. even if that ONE model was any good, which it is not, their entire line sucks for linux support.
get an asus. it was the first company to try shipping linux for real (not dev beta scams) and still pick the most linux friendly components (last time i checked)
I bought a Dell e7240 couple of weeks ago and put Linux Mint 16 on it (since Mint is based on Ubuntu, I wouldn't expect the experience to be vastly different for straight-up Ubuntu). Everything except the hardware Wifi on/off switch works out of the box (the Wifi can be comfortably turned on/off via the Cinnamon network applet). Considering that this is new hardware (Haswell i7, 8GB DDR3 RAM and dual 256 GB SSD), I absolutely wouldn't have any issues recommending Dell for Linux to anyone.
Finally, the above system can also be ordered with 12.04 LTS which means Dell is officially supporting Linux. I don't think it can get much better than that.
Counterpoint: I've run Ubuntu 11.04, 12.04, and now 13.04 as my everyday OS very well and very consistently on two consecutive Dell laptops (the older XPS 13 and a Vostro v131) and both have worked incredibly well.
get an asus. it was the first company to try shipping linux for real (not dev beta scams) and still pick the most linux friendly components (last time i checked)