+1 to what Brad said.
Both my wife and I have used the .30-06 on many "big-bodied" animals in North America and Africa over the years. If there is a non-dangerous big game animal that can't be killed neatly with a .30-06 then we haven't encountered it. In fact the .30-06 has served so well as my wife's "big rifle" on a bunch of animals that she backed down to the .308 a couple of years ago, with no noticeable decline in effect.
You are a lot better off with a familiar rifle. One of my favorite quotes came from a long-time elk guide I know. When I asked if he carried a .375 H&H because he guided in grizzly country, he said, "No, it's to finish off the elk my clients gut-shoot with their brand-new .338's!"
The elk guides I know (and I know a lot) are universal in their opinion that the .30-06 is plenty, and are against somebody buying a "big magnum" because they have heard that the '06 isn't enough.
