I'm not sure how ATI drivers
on Windows Updates compare to those from AMD's website, but couple of things.
1) Drivers from AMD's website would be the the latest you can get. They are released about every month to continually fix issues that come up. If you look at the Release Notes, you can see that the fixes include specific things for specific games, etc. You can compare version numbers between the two offerings.
2) Drivers from Windows Updates pass MS's WHQL tests. They ensure compatibility but may be based on an older driver
version rather than the latest, which means they may actually be one or two releases behind the latest ones. As a result they would likely be missing functionality like OpenGL support and stuff like Avivo.
I normally update only to fix a problem or for a performance enhancement- so, like once in 2 months through AMD's website. I would never update device drivers via windows updates.