I'd pick the one with DDR3 (if available) but I'm guessing they are both DDR2.
I don't think it really matters, but the Athlon quad will move to an AM3+ motherboard when you are so inclined.
Otherwise --- pick the one you can pirate the most parts from for a future build. Compare the HDDs, cases, PSUs & optical drives that you would be most interested in moving up.
C2Q will spank any of the Athlon / Athlon II family of CPU's and overclock nicely.
Maybe in synthetics ....
Otherwise they are pretty much equal.