The FOSS driver and power management doesn't work according to what I read. I'm not sure about drm and I don't know if that's something everyone uses or just 'beta' (experimental) in which you can expect bugs again.
ATI support in Linux is lousy and slow. Bottom line. You only have to read any forum in which ATI cards are used. It's absolute crap. The problem is support for either binary or FOSS progresses at a snail pace and ATI/AMD DO NOT DEVOTE ANY WORTHWHILE RESOURCES to it and people still sing the praises because they toss an open source driver into the mix. I don't consider that support.
Nvidia might say, no one is getting our code but make sure it's working in Linux. I guess it depends on what issues/problems you're willing to tolerate.
Power management is important because it (and the driver) may determine whether you card runs several degrees hotter than necessary and if you bought the card for the lower temps (newer ATI hardware), you might want power management working. If it works with the ATI driver, great, but I'm wondering how well. I have only read your post of the FOSS driver having power management so sorry if I don't share your optimism.