2 full size offers more options and slots for expansion
3 not about rich about odds. if you can get 2x4GB sticks for the same price as the 1x8GB I would.
later getting another stick may be harder than just buying the same model. I have had RAM sticks made one after the next, serial numbers 1 apart not work together in dual channel. if you get one stick now, plan to replace it with a larger set later to ensure they work.