the i7-6700k skylake is not compatible with x99 and the i7-5820k is last gen. dual xeon is promoted/advertised by intel as a way to sell expensive gear, but only works well in special workflows.
the i7-6800k 6 core is good 1080p and entry level 4k. $680 with basic x99 mbd.
the i7-6900k 8 core is good 4k and entry level 6k. $1300-1700 depending on x99 motherboard.
you will want your system builder to do a lite to medium overclock with these to get the best performance, and that can still be done on air with the noctua d15s.
you can use gpu-z to see what your gtx 970 is doing. chances are if you are grading alot and using gpu denoising, especially with davinci resolve, you may find the gtx 970 lacking.
Thanks for the comments. The discussion continues ...
A good compromise between your two suggestions, which I have just built would be:
Intel Core i7 6900K 8-Core over-clocked to 4.0 GHz
ASUS X99-Deluxe II 2011-v3 Intel X99 SATA 6Gb/s USB 3.1 ATX Intel Motherboard.
This strikes a balance between motherboard/CPU from your two original suggestions.
I did pretty much what you are proposing - keeping my case, PSU, GPU (980 Ti), drives, Etc, but as I was coming from X79, I had to use new memory (64 GB of 2666 MHz).
This provides new drive interfaces - including a Thunderbird 3 card, and 40 PCIe lanes for M2, U2 etc.
I only edit 1080p50, but compared with my old X79E-WS and 3930 CPU, it flies.
As RoninEdits mentioned, the 6700K is incompatible with X99 - if only because the 6700K uses a completely different, and totally incompatible, socket from the X99 motherboards. The 6700K uses only Socket LGA 1151 while all X99 boards use LGA 2011 v3. Thus, if you do choose the 6700K, you will need a Z170 motherboard, not an X99 motherboard.
The 6700/X99 combo suggestion resulted from a mis-stated compatibility chart on one Newegg page. Yea, not usable. But a couple other suggestions actually are what my builder was checking on when he realized the mis-match between stated and actual socket for that 6700 chip. So looks like we'll move forward today.
Thanks for catching the mis-match. Builder had been going from a Newegg compatibility list, and realized looking at another page on the chip, that it wouldn't work with the X99 mobo. So ... he'd been looking at other chips and suggested I thank you for catching the mis-match also and for recommending the 6800. He'd been looking at that as perhaps a better-match between budget and performance and with your recommendation, we'll probably go that route.
As to the 970 ... that we'll monitor after the new rig is built. If it's holding things back, well ... that will be next.