I would lean more toward the Quad core with the 6520G. it's a more powerful system with a better video card. It's difficult to say how well it will run D3, as it hasn't been released yet, but don't expect to run Ultra settings on a "budget" PC of any sorts. As long as you have realistic expectations, I think you'll be fine.
Diablo 3 can run on low settings on a dual core i3-2350 with integrated Intel HD 3000 graphics at a consistent 25fps (for a very low budget laptop, that's good stuff). Either of those should be able to do at least that, if not medium, at their native resolution.