Are you making money from this system?
The i7 5280K is a good chip you could expect to overclock to around 4.4Ghz with a good cooler, if you wish.
The i7 6850K is a generation more modern, so will be a little faster, but a little less likely to reach 4.4Ghz. Both CPUs will get 4.0GHz easily (and 4.2Ghz which I see no point overclocking less than)
At school my Computer Club is looking at building a new edit and render computer for the 'TV station'. We are likely to go for the i7 5280. It is significantly cheaper and only slightly slower.
Depending on the software you use, an older Xeon-based system might be just as good, with amore cores and threads to compensate for the slower clockspeed and IPC.