The fx 8350 are typically better binned chips (came out better from the manufacturer) and can overclock better. Overall I think the fx 8320 is a good choice, but since the release of the fx 8370, 8370e and 8320e it is more and more likely that the fx 8320 will not overclock very high. It is a luck of the draw, but AMD is picking out the best ones for the fx 8320e, 8350, 8370, 8370e, 9370 and 9590 chips. The fx 8320 is at the bottom of the barrel.
Having said that, if I was buying an fx chip I would go with the 8320.