What is your budget for your entire rig? It would depend on a few things.
1) Will getting the FX-8320 allow you to purchase a beefier GPU? If so, get this CPU
2) If purchasing the FX-8350 will STILL allow you to purchase a beefy GPU, then get this CPU.
From a few forum members, its been shown that the FX-8320 doesn't overclock as high as the FX-8350. If you're looking for maximum performance (overclocking wise), then 8350 is obviously your choice. If you're looking for a budget-friendly 'good enough' type of rig, then the FX-8320 will be more than sufficient. You could most likely be able to hit at least 4.5Ghz with both of these chips anyway.