The question isn't comparing AMD to Intel. Everybody knows that Intel still has the slight edge. However, AMD has the value card to play, and they play it well.
On topic: The FX series of processors is a new wave of CPUs that blows everything out of the market. They do what they are designed to do better than anything else, and often times for a fair -- if not good -- price. They have been designed to multitask, and that's what they do best. They are of a more expensive series than the Phenoms, and it is difficult to tell if that is partially attributable to the fact that they are newer, or it actually is completely because of their superior processing power.
Looking at AMD's website, it is easy to see that the FX series has the edge over the Phenoms:
http://www.amd.com/us/products/desktop/processors/amdfx/Pages/amdfx-model-number-comparison.aspx
http://www.amd.com/us/products/desktop/processors/phenom-ii/Pages/phenom-ii-model-number-comparison.aspx
However, the question now becomes "Which one is the better value, especially for those on a budget who don't require a top-of-the-line system?" And the answer to that is a no brainer - the FX. At $40 cheaper on Newegg.com for comparable (and actually better) specs to it's Phenom counterpart, the FX-4100 3.6 GHz quad core is hands down the better of the two.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103996
P.S. - The FX series also runs for less power (95W in FX processors compares to the 125W in Phenom processors), and they run cooler by at least 10 degrees Celsius. That alone is a dead giveaway.
P.P.S. - To those who pull out numbers about which one actually runs faster with single vs. multi-threading, there are too many variables to consider. The CPU doesn't work alone, and it's going to be better at certain things than it is at others. The fact of the matter is, every test will provide different results, and there's no way of telling which is better outside the manufacturer's listings, price, and potential usage. that's what it comes down to, and that's the truth.