IMO, the 280AI is the better because it will shoot the heavier bullets. Mine will shoot the 160 gr Accubond at 3000 fps+. That is right up under the 7 mag so the only reduction in recoil you will get will be because the 280AI does it with ~6 grs less of powder.
The 6.5x284 can be a formidable hunting load when loaded with 140 gr bullets which have a sectional density of .287 which is a little more than the 280AI 160 gr bullets which is .283. You will need to get the 6.5x284 in a medium or long action in order to take advantage of those long 6.5 bullets.
I have a 6.5 that will push the 140 gr bullets to 3100 fps but the caliber is 6.5 rem mag and not the 6.5x284. If I am going hunting for anything larger than Whitetail then IMO there is no contest, I grab the 280AI. The 280AI is extremely easy to reload for as is the 6.5 rem mag. I do not know about the 6.5x284. I wish there was only one 6.5x284, it would make it easier to not get confused and make a mistake.