You can't use dual channel without dual sticks, meaning you're effectively cutting your speed in half by getting 1x8.
Not "effectively", no. In practice it does not effect much of a performance hit. I would still recommend getting 2x4 instead of 1x8 every time, barring other considerations (such as if a motherboard only had two DIMM slots and you had a mind to upgrade to 16 GB later, then buying 1x8 now and 1x8 later makes more sense).
-What is best in life? -To crush your enemies, see them driven before you, and to hear the lamentation of the women.