I've been a 284 fanboy since building my 1st one in 2006 - it shot so well in 1000yd any/any prone matches that I bought a BAT 3LL and built another one. A couple of years ago, I did a 284 bbl for my DTA SRS, and it's shot up to par relative to the other two rifles. Then last year, after finding a couple of 5R 1-9tw 7mm heavy Palma blanks on Krieger's online inventory list, I chambered one of them in 280AI, and have been comparing it to the 284s at every opportunity.
So far as I've seen so far, despite the 280AI having nearly 6grs more case capacity than the 284, it doesn't have much to show for it. Velocities have been pretty much the same, using H4831 & SC or IMR7828SSC in the three 284 bbls (one 30", two 28") and the 30" 5R in 280AI. Accuracy-wise, I feel all three of the 284s have a slight edge over what I've seen out of the 280AI so far, but I have to qualify that by saying I haven't had time to shoot the 280AI as much as I'd like, and have a lot more experience with the 284s.
If I had to make the choice between the two cartridges today, it'd probably come down to brass availability. When I built that 1st 284 9yrs ago, WW284 brass in mass quantities was just a call to PV away, and I bought plenty of it. Now it's unclear whether they're ever going to make another run of it, and if they do, when it'll be available. However, Norma's now making 284 brass, and there's always Lapua 6.5x284.
When it comes to the 280AI, you might get lucky and find some R-P somewhere, or take your chances with Nosler and see if the primer pockets will hold up for more than a couple of firings. I've got some Nosler 280 that I've fire-formed in my 280AI, but haven't shot it again yet, so don't know how it'll hold up. I also bought a couple hundred RWS cases a few years ago in anticipation of building a 280AI. Personally, I have a lot of confidence in the accuracy of my 284s, and would probably just stick with what I know, even though I like my 280AI, and will continue to work with it. Until I get more experience with it, it's hard to say for sure whether it's got anything to recommend it over a straight 284.