If you're on a tight budget build, go with the 8320. If you get a decent air cooler, such as the Hyper 212 EVO, you can overclock the 8320 to the speeds of the 8350 easily.
If money isn't an object, go with the 8350. (I would still recommend an aftermarket cooler.)
it really depends on your specific chip, my 8320 is OC'd up to 4.1 GHz, and it still idles at 33c, which is only 1 degrees hotter than it was at stock. Granted, if you get one that binned as not being able to run at 4 GHz at acceptable temps, you will have a different experience than me. But i'd say grab an 8320, it's an amazing chip and you'll most likely be able to get 8350 performance from it with ease while saving $40 to put towards some other component.
More potential with the 8350. If you have the money go for that. They'll both heat up regardless because of the voltage necessary to sustain good clocks.