There is no doubt that ATI has been scrutinized for their Linux support in the past, back when they had seldom driver releases with very few display features. Not only were the display features minimal as well as the frame-rate performance being lackluster, but it was a chore in and of itself to simply install and configure these RPM drivers for all distributions by novice users. Within the past year or so, ATI engineers have appeared to be at the top of their game when it comes to pushing out competitive Linux drivers. ATI has finalized on monthly driver releases, which often contain significant improvements when it comes to bug-fixes, and lately they have also been bringing forth new (and useful) display features.
ATI's Linux push had largely begun with the 8.14.13 drivers, which were released on June 9 of 2005, and had offered a new installer interface. No longer had ATI Technologies simply offered RPMs for each of the X-Windows versions, but they had finally provided an intelligent installer for its Linux users. To this date, ATI does have the upper hand when it comes to the graphical Linux installer. With that, ATI continues to improve its installer by adding support for new distributions. Of the recent ATI features to have enlightened its users include the aticonfig X configuration utility, improved performance, suspend/resume support, PowerPlay support, and the emerging dynamic display management options.
In this time, in my honest opinion, NVIDIA has not been at their peak performance as they have been in the past. After the long-awaited Scalable Link Interface (SLI) support for Linux and Solaris, the benefits of this technology remain poor. There continues to be no SLI control panel options, or additional features within nvidia-xconfig (NVIDIA's equivalent of aticonfig), and from the frame-rate results posted at Phoronix, there is barely any benefit from investing in multiple graphics cards. The release cycle for NVIDIA's Linux/Solaris/FreeBSD drivers has also been stagnant with their approximate 4 month/4 week cycle. The driver releases to commemorate the 4 month cycle have also been fairly basic when it comes to implementing any new features. Even with four-month periods of no Linux driver releases, they fail to deliver any sort of public Beta driver. However, NVIDIA does have the benefit of officially supporting FreeBSD and Solaris x86 with their proprietary drivers. ATI's Linux Beta program does require a signed agreement, but they do welcome any Linux distribution vendor to participate, as well as many members from the GNU/Linux community.
Of course, in an ideal Linux world, there would be no proprietary drivers. However, in this imperfect world closed-source drivers currently remain the only adequate option for running current generation graphics cards under alternative operating systems.