Judging by what you linked, I'm guessing you will want to overclock. If you overclock, you will need an aftermarket cooler, and the tall spreaders on the vengeance ram will interfere with the cooler.
That said, you can get the vengeance in standard profile and still get the same performance. To answer your original question, you will be better off with 2x4 in order to take advantage of dual channel.
Talk to sales at newegg and see if they will do a swap.