Dear Colin,
I agree with everything Tim and Kay said above. i5 or i7 of the same frequency and core count will perform similarly with Xeons (even across all generations of intel core/xeon processors). Adding more cores will help the refining process (under linux and mac only), but unless the computer is being used simultaneously by multiple users, I do not see the cost-effectiveness in running a 36-core/72-thread dual Xeon E5.
As to the OS choice, another interesting point regarding cost arises. Top performance Mac you can buy as of April 2016 is a 12-core 2.7Ghz Xeon (a 3-year old processor), setting you back a whooping 9000 USD. With the help of a tech-savvy student or postdoc, you can easily build something that runs circles around that mac pro with 1/3 of the cost. However you will have to live with Linux, which is not half bad if you get used to it.
I personally still prefer OSX and I run an old dell optiplex with a quad-core i5 (apple won't be happy but it is their fault for not updating their product). It only took me half a day to put OSX and all crystallization software I need on it and everything run like a dream. Due to the lack of performance and high price of Mac Pros, if you want a fast Mac, building a hackintosh is your only choice as of today.