In terms of CPU power, the Core 2 Quad would probably be better due to higher core count, even with it's ancient architecture, though if it's the Q6600 the low clockspeed might also hurt it a bit. The A4 would have better integrated graphics and access to more modern chipset features as it is on a much more modern platform, the big drawback is the A4's platform is about to be phased out and never got any really powerful CPUs, so you don't get much of an upgrade path there.
Neither option is really any good if you need anything more than a basic computer for web browsing, video playback and office work. If you're gaming, the Core 2 Quads can still be somewhat viable, but a lot of them do need overclocks to stay relevant, and finding...