For gaming they will be nearly the same. I5 has a slight edge, but you give up the option for those 8 cores.
If you plan on doing multitasking and highly threaded apps then go with the 8 core.
if you only are gaming, go with 3570k.
EDIT: I mean you also could wait for the Haswell processors to come out, which will be better than ivy bridge
Another bonus to having the FX processor, is that the AM3+ socket isnt dead. You will be able to upgrade to steamroller next year when it comes out.
Both have their perks, and both are great.