The FX "8520" may never come. As of the latest AMD roadmaps, Steamroller FX CPUs aren't mentioned anywhere.
The Intel 8# may has a pretty limited upgrade path since rumors suggest that changes in Broadwell requires a new chipset.
The AMD FX 6300 can be overclocked, but only just so to match Haswell i3s and the completely bottom tier of Ivy Bridge i5s. You can move up to the FX 8320/8350, but that seems to be a pretty modest upgrade.
The Intel i3 is faster at stock, but cannot be overclocked. It can, however, be upgraded to an Haswell i5 and i7 which would not only be a pretty decent speed bump but exceed the power of the FX 8320 and FX 8350 in gaming.