HI Sky
We purchased a refurbished Mac Pro earlier in the year and noticed a huge difference in performance. Very robust, and seems to be able to render all day if needed. Very happy with it, not sure about the new version, I prefer the larger case that allows for easy upgrades. New hard drive in less than 5 min.
All cores are used when renderworks is used, but not during other operations. I've read that our six core unit hits a sweet spot between price and performance.