Navagon: I'm still avoiding Intel because of their wanton mafioso criminal behaviour. I would add Dell to that list for assisting Intel in their crimes, but I don't buy pre-builts anyway so that would be an empty gesture. And no, not a word of that is an exaggeration as they've been legally hammered for it. Truth be told though, the way the market is right now, odds are my next computer will have to be an Intel. Fuck...
Writing this from Xeon machine... AMD is not really that much behind Intel. They went in wrong direction with Dozer because they did not expect single-thread lock to still be a problem in 2013 (where Intel targeted it almost exclusively with DMI bus), and their latest chipset is pretty close on tech to Intel's QPI - where Intel integrated PCI-e since Sandybridge - which combined, gave the known advantage in typical single-thread (render-thread) games.
Navagon: It's been a while since my last AMD/ATI card, but from your post it sounds like their Linux drivers are every bit as good as their Windows drivers*.
*Unless AMD have broken the habit of a lifetime and produced some good Windows drivers in the past few years.
Bits that drive hardware are same, are open and reside in Kernel.
Proprietary driver has only user-space module closed and has technology AMD can't release, and is also designed to support near release-date hardware.
Open driver has user-space module opened and usually catches up with adequate support for 1-2 year old hardware.
In terms of performance, closed is a bit faster.