As many have said, either will work great if you find a good ammo match for the rifle. Just to make it a bit more concrete, I was looking for a specific ballistic comparison. For one example, I was looking at how Nosler loads these two cartridges. According to what they say, for whatever that is worth, in the TG line of ammo, they load them like this with both zeroed at 200 yds: 7mm RM - 168gr ABLR: Muzzle: 2,880 fps. 500 yds: 2,163 fps; -38.8” drop. .300 WM - 190gr ABLR: Muzzle: 2,870 fps. 500 yds: 2,135 fps; -39.4” drop.
Almost identical except for the 22gr/13% heavier bullet out of the .300. Probably not going to make a difference in 99.9% of circumstances.
Of course, there are scores of other combinations. But I think that example illustrates how similar they are. The .300 can push a 13% heavier bullet at about the same speed as a 7mm.
That's approximately true, but the .300 requires about 22% more bullet weight to match the BC of the 7mm bullet, so as distance increases the 7mm advantage increases as well.
