I am not associated with Laminar Research or any other company or organisation within the USA. I live on the otherside of the world downunder and no expert like some others on X-Plane.
I am of the opinion you will get better value for your money if you buy a windows based PC whereby you will be able to upgrade far easier than you will with a Mac System. Although X-Plane is developed on a Mac System, Laminar Research staff have indicated in a previous Q&A answer in this forum to go for a windows based PC.
As I understand X-Plane is currently based around a 4 core system to get the best result. As ice_pdb has indicated more cores will or may be more beneficial in the future as X-Plane develops further. I have seen another video elsewhere (can't find the link) using another operating system other than windows making mention of X-Plane only using 4 cores.
Have a look at the following youtube video produced by MIchael Brown from X-Force PC regarding using multi cored processors. This is the recommended company by Laminar Research for the supply of PCs in the USA. The presentation is based on windows system but it has a message.
I am also of the opinion that your question has been asked on a previous occasion in the same forum. To find the question(s) you will have to scroll back through the pages.
The reference link can be found at https://www.youtube.com/watch?v=ltV8hecd1Lc
Hope this helps.