It isn't; neither one is "real" Unix (whatever that is), not that it matters.
OP: I'm running 14.04 as the only OS on my 2011 MBP and it works perfectly out of the box. Just install it as normal if you don't need or want OS X. Dual booting may be trickier; I have never tried it. You may wish to install TLP for better power management. I can get about 8.5 hours of battery life with the screen at minimum brightness and the keyboard backlight off or very low. Stock Ubuntu power management is not as good.
Otherwise, the wifi is supported, the Intel HD3000 graphics work fine (NVIDIA drivers are available), keyboard brightness and volume keys work, and Trim is supported out of the box if you have an SSD. The much maligned Amazon search thing in the Dash is still present, but can be removed or disabled without trouble.
14.04 seems like it will be the last LTS release without systemd, so that could be benefit or a drawback depending on where you stand on that issue.
Overall, it's a good OS that supports this particular hardware very well. I'm not much of a Linux fan, but I am pleased with it.