Like others have said, it's totally dependent on what model you're using, and the situation it's being used in. You can't compare an X6800 overclocked to 3.2ghz running on Water Cooling and an X2 running Cool and Quiet on air.
If the situations were EXACTLY the same, I'd say that AMD run cooler, just from personal experience.
I run an X2 3800 OC'd to 2.6ghz per core running on Air Cooling and I've never seen it go above 50 under load.
Just the way it works.