The main thing to consider is what your CPU can address. There is no point buying super fast RAM if your core is too slow to make use of the extra Mhz. But likewise, it seems a shame to buy a decent core and not let it stretch its legs a bit.
There will be an equation somewhere that details the differing speeds of high performance RAM compared to high capacity. I would imagine in this case, since you are talking about doubling your capacity and only dropping your speed a little bit, the 8GB should be the faster.