Re: 7mm AM vs .300 WM
There are plenty of reasons to get either. For decades, many who prefer to be a "one gun" hunter choose the venerable 300 WM, and for good reason. It simply works and works well. The hole is bigger, the bullet wt selection is heavier, it is a very accurate caliber. It is an excellent choice.
I do not have personal knowledge of the 7AM other than what I've read here. It appears to be an excellent choice for a do-all rifle. To be blunt, if a 140 Berger VLD from a 6.5x284 can dump a bull elk at 900+ yards, there's nothing either of your two choices can't do better.
If you are to travel across the world to hunt, you can find 300 WM ammo just about anywhere. Not so with the AM. So, if you go on a hunt of a lifetime and your 7mm AM ammo is a no show, you are out of luck. I guess that would be the only downside of any wildcat or semi-uncommon caliber.
Kirby built a rifle for me in 270AM and did an excellent job. The rifle shoots very well.
Good luck with your decision.