The only difference between them is clock speed. The 6400 is 2700/3300/3300/3200/3100 and the 6500 is 3200/3600/3500/3400/3300. So on paper at least, the 6500 is 18.5% faster base clock, 9% faster single thread, 6% faster 2-4 thread.
So far, the only bottlenecking being reported here has been with 1070 and faster cards, and only in extremely CPU intensive games at 100+ fps. The 6500 is probably a better value, but, the 6400 is a very capable CPU.