I don't think there is enough of a performance difference to justify an upgrade from an 8120 to an i5. At very least I would say get an i7 3770 but if I were in your position I would wait for Haswell or go SB-E.
Haswell might not be the light at the end of the tunnel, Intel have no reason to make it better atm.
Until Piledriver launched, I would have thought the same. But maybe it changes things? While they've not caught up to the i7 in total performance or even the i5 for gaming performance, they've narrowed the gap by quite a bit. Maybe this will cause Intel to pull out one of the many tricks up their sleeves.