Interesting article about how green those Western Digitals really are:
That said, I would go with something FAST. I'm not sure what's on the market now, but there's all sorts of cool technology coming out: USB 3.0, light peak/thunderbolt. And not only that, but also a fast drive speed, they're listed in rpm. (note: you'll still be limited by the speed of the drive you're currently using)
If you're like me, you probably hate sitting there waiting for your data to transfer back and forth. I have two older model 1TB each Western Digital drives, and it's seriously the amount of time it takes to transfer data that consistently holds me back from constantly backing up my data.