They should have the same overclocking ability, I would myself take the 8350 because of improved architecture.
Intels doesn't have anything that can garantuee a stable 5GHz, but GHz have little to say about actually performance.
Piledriver have a longer pipeline, and with a longer pipeline comes higher GHz, but wont actually have any performance improvement than one with a smaller pipeline and and lower GHz.
Haswell is a small lottery, when it comes to overclocking. You can get a good chips, which can have good overclocking ability, and then you can have a bad chip, which can have bad overclocking ability.