what do you need the computer for? multi threaded applications?
instead of getting a dell, i would built my own computer. the parts would be of higher quality and also would leave room for overclocking and upgradability.
also your link for the option b doesn't seem to work.
If your application scale linearly, the octo core rig would certainly be far better. Use a real cpu cooler though, not that passive one, if you don't want to burn out those cpus.