If you were doing new board, the real world difference between them isn't worth $100. Review websites by their own admission, use gaming scenarios that are designed to stress the cpu in ways that are largely not reflective of real world performance.
Last january, tech syndicate opened up a whole can of worms when they tested the 8350 against the 3570k and the 3770k and the 8350 beat them both 90% of the time. Even they were stunned. But tech syndicate doesn't use synthetic benches and uses only real world situations. So its not as big a gap as most users and reviews would suggest. And the FX is a whole lot cheaper.
All that said, you have the board. Your saving a lot of money there. 3570k is a no brainer in that scenario.