You are presented with a 300 yard shot at a world record elk. You have at your side a 243/6mm and a 300 win mag. You shoot both equally well. Which are you gonna pick up? Should a lowly cow get any less respect?
What would be the difference, if both are loaded with TSX? The bullet will penetrate plenty from each.
I'd pick the 243 because I know that I would shoot it much better than the 300 and have much more confidence in it.