Depends honestly the A6-6400K is slower than the A8-5600K as the A8 has 2 more cores and more gpu cores. The main difference between the two architecturally is the the 6000 series has a better turbo core functionality. Now as far as minecraft goes it has very low requirements but if you want to bump the settings up to max and use graphical mods and/or shaders then even an A10-7850K wouldn't be sufficient. So as far as this goes the A8 would give better performance.
If you were comparing the 6600k to the 5600k the 6600k is the clear winner. Also when using an AMD APU make sure you are using 2 Sticks (same size memory) of DDR3-1866 or faster. AMD APU's are much faster when paired with fast memory.