Both the 6800k and 5820k are similar. An 8 core CPU may help more if you can afford it.
I'll personally wait for the Skylake-E because the 3570k is not so slow and you'll get new standards like optane, Thunderbolt over USB-C, AVX-512, etc.
If the choice was the 6700k and 5820k, then selecting the 5820k will be obvious.
Also don't bother overclocking/undervolting the 6800k or 5820k because the the power management and automatic throttling are fairly good.