Always go for the 2 sticks if ur intending to push the system with help from Mr Bios.
Matching 4 sticks will work just as well as a matching 2 sticks, or a kit should I say, but 4 sticks can demand that little bit more from the mobo in order to be stable whilst being pushed.
Without pushing, or oc`ing a system tho, either would be fine. Here u would look at the timings and perhaps make a decision. Chances are tho, if ur gonna push ur system alot, u`ll slacken the timings anyway - even tho it`s good to work from a good timings base.