by the white papers dual channel ram will be the better performer. By having the memory run in dual channel it will have a greater bandwidth and lower latency.
in practice however there isn't a great deal of difference for people that use off-board video cards, especially for AMD systems that don't make use of the extra bandwidth.
Some backstory: Dual channel ram was actually developed for the mom and pop computers that use onboard video. This onboard video actually uses the system ram, now having both a cpu and a video card vie over the ram using only a single memory controller, things can get bogged down pretty quick since a memory controller can only make one call at a time. With the dual channel ram, there are two independant memory controllers that can each make calls, so when the video card and the cpu both try and pull the space out for use they can do so simultaneously.