The 3xxM is the older Arrandale architecture, while the 2xx0M is the newer Sandy Bridge architecture. As far as I can tell, both are dual core, with integrated DDR3 memory controllers, Direct Media Interface on the processor, integrated GPU (unused on your laptops?), 256 KB L2 cache per core, 3MB shared L3 cache, and have 35W thermal design power. About the only difference might be Sandy Bridge support for Advanced Vector Extensions.
I'm guessing, but maybe the newer/slower CPU is more expensive just because it's newer. Given the virtually identical specifications, about the only differentiating factor seems to be clock speed. I'd expect the older CPU with the faster clock to perform proportionally better than the newer CPU with the slower clock.