maybe a little more info to reflect upon..best combination if you have to mix would be 5300 that can outperform it's spec with 6400 memory that is to spec, if the goal was for example, to run all sticks at 6400 performance levels. for example.
i have mushkin 6400 that i put in my box to upgrade from corsair 5300. i then decided to mix both, but, if i was going to do so, i wanted all sticks to run at 6400 or better levels. fortunately, corsair is in my experience, always able to outperform it's specs (at least the xms2 sticks or better, maybe not the value ram) and sure enough, i was able to easily run all sticks at 6400 or better and using the spec tight timings of the mushkin ram on both (4-4-4-12)
on the opposite side, i wouldn't try to take for example, ocz 5300 speced memory and mix it with mushkin or corsair 6400 because the ocz in my experience and in reading, tends to be exactly what you're paying for in terms of specs without much headroom beyond on some of their brands of sticks, anyway.
(not a criticism , ocz lovers..nothing wrong with getting what you pay for..but if i can get more, than i will)