I would go for the FX-6300.
It's higher frequency 3.5 GHz vs 3.1 GHz, the cores are the faster piledriver, rather than the slower/older bulldozer, which gives better IPC.
The FX-6300 potentially uses less power 95W rather than 125W.
Even if you used all 8 cores of the 8120, I doubt it would give much speed benefit to an FX-6300.