depends entirely how youll use it, ill save you sifting through hundreds of threads already like this
for just gaming, the intel for now will wipe the floor with the FX and at considerably less power consumption.
For light multitasking, and heavy single threaded programs, the i5 will run better.
for streaming or recording gameplay, the extra cores in the FX will give it the edge over the i5
for rendering 3d images or animation, or traditional video edits, the FX will decimate the i5.
As a gamer 20% of the time and a 3d modeler/renderer 80% of the time, the FX 8320 was an obvious winner for me. And don't get me wrong, I play the AAA games and my FX 8320 has never had a problem holding 50-60 frames at high settings. MY HD 7870 GPU on the other hand tends to hold me back bit these days.
Update: to specifically answer your question the i5 3670k will see about 50% performance per core over the FX. but the FX has twice as many cores, and that's why these chips tend to be neck and neck. the rest will boil down entirely to application/use.
+CTrurbo is notoriously biased towards intel. I love both honestly.