Well the i7-5960X can generally be overclocked to 4.5 GHz. I have it and am delighted with it. Even if you do not plan to overclock see below. The only advantage the Xeons have is that they can be paired but your motherboard does not have the capability for 2 CPU's
Also here are the specs on the Xeon E5 2640 v3
Processor Base Frequency 2.6 GHz Max Turbo Frequency
Here are the i7-5960X specs
Processor Base Frequency 3 GHz Max Turbo Frequency 3.5 GHz
Thanks Bill for your input, much appreciated. I know the ASUS x99ws does not have 2 sockets but I thought about using the ASUS Z10 D8 ws board as it has dual sockets.
Although cost is a factor, speed and stability is more important and I'd rather spend more money to get a system that is the best I can get and that's why I've considered going down the dual processor route. I'd rather spend more to get a system that will work with all the other components and get the best out of Premiere Pro and After Effects as I'm working on projects that combine a number of different formats (DSLR, GoPro 4k, AVCHD etc) on multicam sequences with effects and colour grading.