I second Yooper's thoughts on the barrel length. 24" tapered a much better balanced package. As for the caliber, both are great. I have used .270's for many years. Not shot the .280, but I had a 7 X 64, which is virtually the same caliber as the .280. For me, the advantage goes to the .280 as it can shoot heavier bullets (175 gr.). But, as others have pointed out, there are trade-offs with respect to ammo availability and bullet weights offered. If you plan to reload at all, I definitely would go with the .280. Again, as Yooper suggested further on, if you do get the Pro-Hunter barrel, I would have it shortened.
Pierre