I give the nouveau devs a lot of credit for the impressive work they've done, but it's definitely not (and never has been) on par with the radeon OSS driver.
For proof, just look at this very article. Michael mentions wanting to test on more cards, but that these were the only ones that actually ran correctly. There's been a lot of effort put into the r300g and r600g drivers to ensure they work on the widest array of hardware possible. Stability and features like power management are ahead there.