If money isnt an issue, go with the 2x8. down the road, you can add 2 more sticks in and youll be set for life with ram.
however, the 4x4 is cheaper, but you cant add any in the future.
if you run heavy programs and if only 16gbs isnt enough, then go with the 2x8. most people dont need more than 16gb so if you are one of those people, then go with the 4x4.
look around and see if you can buy faster ram, like 1866. hope this helps