i7 5820K (2011v3) 6 core vs. i7 6700K (1151) 4 core

Ideally, you want fast clock-speed *and* more cores.
You really don't want to sacrifice one for the other.
(If it's a large difference in clock-speed, I'd go with fewer cores running significantly faster.)
 
The 6700k is barely hitting the street.
I'd sit out the first gen motherboards... and let the issues settle.
 
You'd be very pleased with a six-core 5820k running at 4-4.5GHz.  
 

Funny, I got back from buying an I7-5820K at Microcenter like 10 mins ago. I did a lot of research on 5820 vs 4790 vs 6700. If you want a future upgrade path (no guarantees) go 6700 (1151 skt).
If you want a cheap but powerful system go with 4790 (1150 skt obsolete)
Since my new studio PC build will do nothing else but audio and the 5820 (2011 skt) may have one more  generation of upgrades with broadwell or the 8-core I7-5960X ($850+), I went that route. I intend to overclock the 5820 up to 4.5Mhz (@6 cores) if I can get that high.
For the 6700 or the 5820, you pay more for DDR4 RAM and maybe a bit more for the motherboard than with the 4790.
My 4790 build estimate was about $1100. (32GB DDR3 RAM, 2 SSDs/1TB HDD w/o video card, $60 cooler and $70 case).
My 5820 real build (32GB DDR3 RAM, 2 SSDs/1TB HDD w/o video card) is about $1550 of which $110 is the OC cooler and another $140 for the case. The DDR4 32GB was $259.
So $200-300 more for the 5820 vs 4790 depending on case and OC vs no OC. You could certainly put together a very respectable 4790 system for < $1000 even with video card.
I did not do a 6700 estimate since I cannot find one I can get my hands on.
 
If you are worried about a new motherboard supporting a future processor, no one really knows.
I would expect the Skylake 1151 socket to last a least a generation or two, but Intel has not told us what their plans are for future sockets past Skylake.
For Sonar or other multithreaded DAWs, I think more cores is slightly more important than raw clk speed but both is better still. Unless you are using multiple monster samplers or heavy duty multiple synths with tons of VST effects, the 4 core CPUs (4790, 6700) will be more than enough.
BTW, Microcenter has 5820 for $299, 4790 for $279 (I think).This is $90 off 5820, $60 off 4790. In store only. I could not find anyone with the 6700 yet but I read it will be ~$400.
The remaining parts for my build will arrive Thu, I will post how it goes.