The i5-3450 has a 3DMark11 Physics score of 5290, whereas the FX-8320 has a physics score of 6180 so it's a tad faster in that respect.
That doesn't paint a whole picture though, and you should know that the i5 has fewer cores, but they are stronger, whereas the FX-8320 has more cores, but they are weaker. That means the i5 has better single-threaded performance and the FX-8320 has better multi-threaded performance.
In Crysis 3, Far Cry 3, and Battlefield 4 (which take advantage of as many cores you can throw at them) the FX-8320 comes out on top most of the time. However, in older games and applications that don't utilize multi-threading the i5-3450 wins out most of the time.
So there you go, both are fantastic processors and each has it's strong suits.
Go with the FX-8320 if you use a lot of multi-threaded applications.
Go with either the Haswell, or Haswell revamp for an Intel system, because the support and upgrade options for the prior Gens are minimal at best.