It's a quad core CPU.
But the cores share resources.
They've all got an integer unit but they share FP units.
So, it's not a Dual Core CPU with multiple threads. It's a quad core CPU with incomplete cores.
And now to answer your question, go with the Phenom if you're going to overclock.
Otherwise go with the FX.
BUT, if you can afford a FX 6300, I strongly recommend you get that instead.