Your comparison doesn't quite follow... yes it is better to have that, but you're comparing 12 cores at a higher speed than 6 cores at a lower speed. I'm not sure if there is an advantage of having 2 dual cores vs. 1 quad core. It would be more expensive to have the 2, since you'd need 2 motherboards, even though you have the same number of threads.

I'd be interested to hear what material you're reading, I've looked for info, but I haven't found that much on setting them up (in terms of hardware). I think I read somewhere that you want to have as close to the same specs for each machine as possible, but I don't know if that has to do with the distribution software or what.