Everybody who thinks that a Core-i* CPU and its equivalent southbridge are just as good as a Xeon CPU should perhaps take the time and read this article:
http://danluu.com/cpu-bugs/
While the article does not explicitly mention whether Xeons undergo a tougher verification process, comparing the number of know bugs in the
E5 v4 and the
6th generation Core-i seems to favor the Xeons. Part of the rock solid stability of the cMP can be attributed to the fact that Xeons are more mature than their Core-i counterparts.
If this is worth the additional costs? Up to now Apple made this decision for you. When you wish for Core-i processors in a MacPro, please consider that you might not only get higher speed for less money, but less stability, too. So choose wisely.