Since android was started, phone developers have pushed it in directions that Google didn't originally plan for. That's why the nexus s only had single core, and afaik, all the dual core phones have software on top of android to manage the dual core processing, which doesn't really do much for them. yes they're faster, but i think not as fast as they could/should be.
i'm assuming the next nexus will be a dual core, and with android that has support for them. if so, it'd blow all dual cores away to this point, because processor management is more efficient the lower in the stack it's handled.
however, what with the nexus s 4g being recently released, i'm not expecting the next nexus to be around anytime soon as G focus on tablets.