Well, they are really close to each other. I have a 8320 personally. I think it works great and is really fast.
Here are the factors you need to know:
1. Do you multitask? If you dont do alot of it, 8 cores is not needed.
2. To you game? If you do, the 4670K is better.
3. Will you overclock? Both these chips can be overclocked, but the intel one will be cooler because it runs at a lesser wattage.
Overall, i would go for the 4670. The only reason i got the 8320 is cuz i got it for $80 on black fri.