I preach that the best gun you can use is the one you have supreme confidence in. I use a .300WM for everything. I select different rounds for the different big game, but all in the .300.
When I was guiding I remember a client came to camp with a .338 for his mulie hunt. We all razzed him asking, "you want to gut him before he hits the ground?", or, "You don't plan on eating any of it do you?" Fact is he did no more and actually less damage than someone with a much smaller caliber might have done. Matching the bullet to the game, I think, is more important than gun to the game. After all, a bigger bullet isn't going to open up and do damage to a smaller bodied animal.