"ATI has longer internal testing phases" - yes they are so long that they need 18 month to fix some bugs.
or the fact that i had to try three different versions of the official nvidia drivers on my moms computer to find one that wouldn't cause it to periodically lock up.
In some cases nvidia drivers do not fully work correct, but you can at least ask questions in nvnews.net and see if some devs will look at the issue. In most cases those are not known or hard to reproduce. Also you do not have to wait from month to month for next drivers, but they are released if needed (can be longer than a month too).
Probably two Gallium drivers will only differ as far as the hardware implementations of certain functions (eg. rasterization) differ, since all hardware is running using the same state engine. So there will probably be less variability. But right now, very few applications are being designed with open-source drivers in mind at all, so we'll have to see how it goes.