Most games don't need 8 cores or utilize them. Either cpu will work fine; the 4170 should run cooler so you may be able to overclock it higher. If you want to run alot of background stuff such as downloading dvd's while you're gaming, then the 8120 may be a better choice. But for gaming only, I'd go for the 4170.
For gaming which of these CPU's would be the best deal.
Also Plan on overclocking with a Hyper 212 Evo.
between the two, the 8120 is the better overall performer. afaik, it's got more integer cores, similar oc potential with a decent cooler. all fx cpus are harvested from the same die. 8120 is a lower clocked 8150, 4710 is a high clocked 8150 with defective cores lasered off.
for overall gaming, though, intel sandy bridge and ivy bridge core i5 cpus are better than fx cpus.
Although the way they actually work is very different, I think it's fair to say that each Bulldozer Module is basically the same as an Intel core with Hyperthreading (just with less performance overall).