I bought a box of Nosler .280 Ackley brass last time I was in Winnipeg. The sticker shock was harsh; Lapua brass is reasonable by comparison! But I wanted a few so I had a basis of comparison for my fire formed brass, so I sucked it up. You'll find that .280 AI dies are expensive too, so if you go that route, over say a 7 Mag, do it because you really want that cartridge. That was my excuse, now I'm just waiting for the rifle. The funny thing is I have much more 7 mag brass, than .280, and I don't even own a 7 mag . . . at the moment.
A .284 chambered in a long action rifle has the advantage of lots of magazine length for seating out long bullets, but I prefer long cartridges to short, so between the two, the .280 would be my choice. I doubt there is much to choose between them in terms of performance, but in terms of performance, a 7 mag trumps the .284, the .280, and the .280 Ackley while adding nothing to the weight of the rifle, or to the cost of shooting.