Radeon VS ATI proprietary.
Since I installed Slackware64 v.13 I've been using the open source Radeon driver. Mainly because it recognized my monitor's ability to display in 1280x1024 where as the ATI driver could not and I was stuck in 1024x768.
I recently upgraded my monitor and tried the ATI driver again, and it was able to display up to 1600x1200 on the new monitor.
In comparing the speed, I don't know why anyone would want to stick with the open source driver. The ATI driver provides better GPU speed...well, not just better, FANTASTICALLY MORE SPEED than the open source driver. With the open source driver, just trying to play videos full screen was a chore. Massively skipped frames were the norm. With the ATI driver I've been able to play HD quality video without skipping a beat.
My Slackware PC Experience is finally above Windows.