i'm looking to build a new computer
mostly for Video editing ( usually in after effects )
but which one have better performance in aftereffects?
6700K or 4790K ?
DDR3 or DDR4 ?
Well of course a 6700K will be faster than a 4790K, clock for clock. Even more so if you'll use high freq DDR4 vs DDR3. You can use DDR4 3000+ for Skylake vs only DDR3 2133 for Haswell.
Have you considered the hex-core i7 5820K, on an LGA2011-3 board with DDR4 memory? It is about the same price as the Skylake and, with its extra cores, will process faster.
I use Trident X at 2400Mhz on my 'big' system, just fine.
This is the order of performance for editing. The Cooler Master EVO is a 'place-holder' cooler to remind you that for overclocking, which you will likely do some of, you need one, and in the case of the Skylake, it is compulsory.