If you subscribe to the thought that it takes 1500 FP of energy to cleanly kill an elk, I think the only one that gets you there in factory ammo for your 750 yard distance is the 28 Nosler. With handloading, I think the 6.5 is the only one that won't get you there. If I HAD to have 1500 FP of energy at 750 yards, I'd go with the 280. It will get you there with less fuss.
I think with modern scopes, modern high BC bullets, great range finders and the nature of the internet, folks think shooting at 750 yards is fairly easy. It is much more difficult than shooting under 500 yards. It seems folks first focus on bullet drop. That is not the most difficult factor to manage with known range and known bullet BC. There is no real guess work unless you are shooting up or down at an appreciable angle.
Wind drift essentially doubles between 500 and 750 yards. Wind is not constant. Western terrain can do weird things with wind. Wind causes more misses than drop. Duping the wind is a challenge, and it's not one that we can easily train for.
I chose the 6.5 PRC for my western rifle, and I have chosen 500 as the max range. I will never have the time to do the load development and get in the practice needed to get good at ranges beyond 500. Long range shooting is so much more than a good dope card, but the net has folks thinking it's not. I don't think I have ever spoken directly with someone who fancied himself as a good long range shooter who could spit out wind drift numbers or even do a good job of estimating wind speed or value. I don't socialize in long range shooting circles. I am certain competitors in long range shooting disciplines know their stuff. The average guy on the internet or at the gun counter, not so much.
That's a long way of saying choose the PRC and limit your maximum range to 500ish. If you are dead set on 750, go with the AI because it will be much more pleasant in doing the training necessary to become proficient at 750 yard shots.