Reading Anandtech/HOCP/TechReport and a few others gives the general impression that quad is starting to supplant higher clocked duals in games as well as multithreaded apps, and as these are the only things that really need to be sped up, then there isn't really a reason to go with dual core. HyperThreaded dual cores are more like quads so will ignore those.
If something only runs for a couple of minutes (MP3 encode), who cares if there is a 20 or 30% slowdown from going from dual at higher clocks to quad? On the other hand, if you want to convert a DVD to an MP4/XVID/DIVX, you'd be crazy going with a dual, so in the cases that matter, quad all the way.
In the power department, yes, Intel chews through less 'lectrickery. But unless you fold 24/7, you aren't going to make the money back given general usage. So I'd go with the AMD solution on the low end, and either pocket the $$$ or, as Merc suggested, use the difference on parts that will make a difference, like an SSD, a blu-ray player instead of DVD, 80+ power supply, more memory or upgraded video.
Merc, next time your in the buying mood, pick up an Athlon X4, 4GB of low latency DDR3 1333 (only a few dollars more than the C9 stuff) and an 880 or 870 based mobo from Asus, Asrock or GB. You'll probably be surprised how snappy it is. Sometimes the Phenom II is a fair bit speedier than the X4, but generally I don't think it justifies the price--and the people buying these systems wouldn't notice the difference either I'd guess.