I was present when Bronco was shooting his Ultramag last weekend and it was quite a cannon. While Bronco doesn't have tons of experience, he is clearly a "natural" shooter because he handled my P-14 ".303 Winchester Magnum" like it was a .243, so he is no recoil wimp by any stretch of the imagination.
This just goes to show how much harsher the recoil on the Ultramag is compared to the ordinary, old-fashioned .300 Winchester Magnum.
Personally, I consider the Ultramag to be a specialist cartridge, designed for game shooting between 600 to 1000 yards. Since I don't plan to shoot game beyond 600 (and inside 600, my .300 Winchester with a 180-grain Accubond at 3,060 is more than up to the task, especially since I use a mil-dot scope and a laser rangefinder -- and I have a fair amount of experience shooting competitively at 600 metres), I don't personally have need for the Ultramag. I respect the hunters who can really wring it out, though.
No matter what anyone says, this is NOT the rifle for a novice hunter or shooter. Like I said, if you plan to keep your shots inside about 500 or 600 yards, the average shooter would be much better served with a .300 Winchester or 7mm Remington Magnum. Frankly, if you plan to keep shots inside about 350 yards, then a .30-06 or .308 with a 168-grain TSX is more than enough. How many of the AVERAGE hunters out there can realistically connect on game beyond 350 yards from field positions? (I do NOT consider guys like Stubblejumper to be anything close to the average hunter in terms of ability, experience or hunting techniques. Guys like Stubblejumber are who the Ultramags are made for -- true long range hunters. Most of us will never reach that level.)
Having said this, if I had to choose an Ultramag to use, I'd personally go with the .338. It really is better "balanced" ballistically than the .300, in my opinion. It's actually very close to the .338 Lapua, which is the current king of long-range sniping and Canada's long range military sniper cartridge.
By balance, I mean that the powder charge is not as disproportionate to the bullet diameter and weight as is the .300. (The 7mm is worse yet, and that is why nobody talks about it much anymore.)
I don't see a significant trajectory difference between the .300 RUM and the .338 RUM, assuming you load proper VLD bullets, and I believe that for real serious long range work (i.e., 1,000+ yards) a heavy, streamlined .338 will defeat the wind better than a .30, and will shoot just as flat, if not flatter. It will also hit harder and make a bigger hole. In addition, I'd expect that barrel life in the .338 would be slightly better.
This is not to slag the .300 Ultramag in any way. I just think that the .338 is better balanced and hits harder -- for those who can handle it.