A great article on the state of Linux GPU drivers, great comments too. In summary, Nouveau is now being fed documentation by Nvidia. Nvidia has had and increasingly continues to have the best proprietary GPU drivers, both in terms of performance and reliability.
Meanwhile AMD's limited but more willing support for fglrx makes it the perform better with free distros and perhaps cutting edge ones with newer kernels. Proprietary Catalyst drivers are closer to their open source counter parts, but have suffered from crippling 7+ month gaming performance issues.
Practically speaking, if gaming is your first priority, and you don't ming being behind on the latest kernels/distros, or the very real possibility of exploits in the driver binaries, or their harsh controlling stance on open source, Nvidia is your first choice.
If gaming is an after thought to playing 4K video and juggling 20 animated windows and chrome tabs, then AMD's approach would allow you to comfortably use open source drivers exclusively, with the option to go with Catalyst drivers once Linux gaming gets taken seriously by Valve.
P.S. as a side note, today once again Microsoft announced they are holding on to dear life by suing Kyocera over Android related patents.http://arstechnica.com/information-technology/2013/09/nvidia-seeks-peace-with-linux-pledges-help-on-open-source-driver/