The title sums it up, what do you think of a Xeon E5-1620 V3 Quad core being used on X99 instead of the i7 5820k?
The i7 has two more cores and an unlocked multiplier. Also, it has integrated graphics, but the Xeon doesn't.
I'd immediately recommend the i7, although I prefer AMD. AMD's Xeon chips were made for servers, although they would work on for a normal PC, there will be some issues.