well they're both made with the same cores. meaning at the same clock speed each will perform the same in single cored tasks.
they both will overclock well, most games won't use more then 1 or 2 cores, so the higher you can overclock the chip the better off you are. That said since the 8320 has 2 more cores on hand, in the few games that will use every core you got, you'll see it almost 33% better then the 6 core 6300 at the same clock speed. so it has a lot more power under the hood.
If picking the 8 core will cause you to skimp on the motherboard and cpu cooler, i'd get the 6300. if money isn't really an issue get the 8 core.