When it comes to making a choice between a 284 Win or 280 AI, there are factors in each ctdg's favor. First off, a 284 feeds very reliably from AI 300WM magazines in B-O M5 DBM, w/o having to do any tweeking to the mag's feed lips. I've not had much success getting my 280 AI to feed reliably through these mags - even after tweeking the feed lips. I hate to screw up $88 magazines that work perfectly with the 284, so for now, I'm waiting on Accurate Mags to come out with a new magazine for the 30-'06 family of ctdgs.
WW284 brass is scarcer than hen's teeth right now, so you'd have to spring for the more expensive 6.5x284 Lapua and neck it up. R-P 280 brass is also hard to come by, but you can still find Nosler 280 or 280 AI. Personally, I think the 280 brass's primer pockets may last longer than a 284's when run with loads developing the same pressures. Seems to me that the 284's rebated rim design promotes loose primer pockets, but I have no scientific proof to back up that opinion.
My 280 AI is built around a trued Rem LA700 with a 30"Krieger 5R hvy Palma 1-9tw, and though I don't have nearly as many rounds through it as I do through the 284s yet, it appears to have accuracy about on par with the 284, which is excellent. The 280 AI will probably have a slight velocity advantage, but it should, since its case capacity is nearly 6grs greater.
So if mag feed is of no great concern to you, the 280 or AI may have an advantage due to brass availability. And if Accurate Mag ever does make good on their promise to make AI-style mags suitable for the '06 family, the magazine fed issue will go away.
I've shot several 1000yd any/any LR HP matches with a couple of my 284s, and while recoil w/o a brake (not allowed in the NRA's HP rules) gets a little tiresome after 75rds, it's still tolerable. My rifles each weigh 15.25lbs, and with a heavy shooting coat & sling, I'm still able to focus on a clean trigger break at the end of the day, although it takes more concentration than when shooting something like a 6 Dasher. That's just my personal limitations kicking in - another shooter might not have any problem with the 284's recoil. I've got Harrell's tactical brakes on both 284 & 280 AI bbls that I'm shooting now, and they make a lot of difference, especially when shooting off a bench. They're both fine cartridges for your stated purpose - it'd be a hard choice for me if I were starting over, having experience with both.