If I may refer you to this link, it explains it pretty well.
http://www.tomshardware.com/answers/id-1779716/8gb-ram-stick-sticks.html
Basically 2 X 4gb is faster than 1 X 8gb.
However, keep in mind that you will then only have 2 more slots open for upgrading further, which maxes you out at 16gb..