It's always easier to overclock using 2 sticks of RAM.
Most the world records for overclocks are on motherboards with circuitry that is physically smaller in dimension(s).
The reason for this is that the electricity / electrons have less space to cover between points within the computer hardware, as such you can force the buses to switch at a faster speed (clock rate in Hertz).
Extra voltage also helps transistors switch faster, however the heat generated does not follow a linear increase with clock speed AND voltage. As such you need to dissipate that extra heat.
Make a shortlist of say 5 to 25 dual-channel kits you are looking at and we can critique each one depending on your goal. (e.g. high memory overclock with 'slow' timings, or moderate memory overclock with 'faster' timings, etc.).
All companies that sell memory will have very good packs (usually) and they ALL have some very terrible ones on the market too, especially for the asking price as it's a buyer beware market.
Merely by asking you've put yourself ahead of the pack.