CPU Selection

Hi, im upgrading my main computer, from an old i5-3570k and 16Gb of ram.
I’m upgrading only the MoBo, CPU and ram since i’ve got the opportunity to sell some components for a good price.
The rest of the rig:
600W OCZ (72Ah on 12V)
Samsung 840 Pro 240Gb
1Tb caviar black
Asus GTX 780 DC2OC.
I was pretty satisfied by this rig and the gpu works veeery well, the only thing i was worried was the speed on how UE4/VisualStudio compiles both C++ projects or just lightning and building, i’ve assumed it was the small cpu.
So now that i can choose a new cpu i can’t choose correctly for an i7-6700k with an Asus Hero viii and 16Gb ram (2x8Gb) or an i7-5820k with an Asus X99-A.
the only difference is the number of core actually so my question is, does compiling, building and rendering works better with 2 extra core and 600mhz less?

My main work is to program BP and C++, some times maya modelling but not so heavy, also i can’t affoard more than a 5820k. Also thinking if a Xeon was worth around 350€ but then i don’t know if ECC memory is needed and also a WS motherboard is necessary