go with the i5, there is a whole lot of complex computing reasons, overclocking THEORETICALLY gives linear performance increase, but in games this is NEVER true. Because games do not issue a complete data set and let the CPU decide how to go about finishing the calculation. Games work on an extremely linear instruction set where the game engine issues specific instructions in a specific order. Because of this the next command in the list requires the output from the first command. This means that IPC is far more important than clock rate in determining performance.
Basically, the i5 will out perform the 6300, even if you overclock the 6300.
This is based on having a steady frame rate. If you average the frame rate over time, there will be times where the fx is issued istructions it can handle more easily, but what you will notice is that during each second the i5 will far out perfrom the 6300
A weighted average of FPS over play time will showthat the i5 is FAR SUPERIOR. Just get it.