If you have a good 270 Win., you are capable of handling any game in north america, except the big bears in close quarters. The only reason I could see going bigger, is if you want to hunt grizzly bear on up. For that task, the 338Win. mag is an excellent choice. If you want only two rifles to hunt world wide, it would be hard to beat the 270Win. and the 375H&H mag.
That's my two cents, and probably worth a little less.