I shot in this comp with a 300 win mag. It was at 800,900,&1000 yards. Started out with 185 McMillan ammo then got 190 from AMU. The gun did everything I asked of it. I don’t have any experience with an ultra mag so I can’t help there. In the right hands I can’t imagine there would be much of a difference between the two. I don’t reload either so the big decision for me would be which ever one is cheaper to feed. I’d also think about the distances you’ll be shooting at, if it’s only 300 or 400 yards I think you’d be leaving a lot of potential out. To each their own though.