Getting back to the hyper-threading issue... FreeNAS will not benefit from HT, but a 2 cores would be nice to have. There isn't much in FreeNAS from what I've read that is multi-threaded so as long as you have a few cores, that is all that matters with respect to you specific question.
As others have pointed out, there are other factors in purchasing a CPU and these impact the overall cost and performance too.