I guess the powers-at-be also had to justify spending a significant amount of money on Kirknewton to improve the airfield surface, drainage etc. Although Arbroath is a larger, flatter and drier airfield on an established military base complete with the facilities that go with that (security, messing etc).
So, in terms of a site, Arbroath is preferable, but on paper Kirknewton is probably a better location for the masses. Not that this is much consolation for those in the north of Scotland.