I was thinking about this earlier, does anyone know how hot GPUs run mining bitcoin? If that runs cooler I would tend to think the extra power draw and heat is because scrypt pretty much rewrites the entire ram available as fast as possible over and over again, whereas Bitcoin/Darkcoin would only use a small amount of ram.
Curiosity got the better of me... below are some test results for 2 x HD7950. Power is measured at the wall. Idle power is 210W.
darkcoinhashrate: 3.80Mh/s
temps: 52,57
power: 415W
bitcoinhashrate: 1.21 Gh/s
temps: 69,76
power: 680W
litecoinhashrate: 1.34Mh/s
temps: 73,81
power: 740W
This may not be a valid comparison because I don't know how to optimize settings for bitcoin. All I did was fiddle with the intensity setting. Does 605Mh/s for each HD7950 sound reasonable?