It all depends on what you are building the rifle for. For extreme accuracy, formidable terminal ballistics and long range applications, the 6.5 Grendel from Alexander Arms is unbeatable. Under 500 yards the 5.56 would be my choice, over 500, the Grendel because you have a much larger choice of bullets to sort through as you reach further out.
How does the Grendel compare to a 308?