308 is just as fine for elk as the 30'06. As bigbulls said, they are very close in performance, what one can do, the other can to. That being said, I would be less confident on moose, and would not push my luck with bison or bear of the non-black variety. Keep whichever gun you like best, and sell the other, if you like, or keep both. For the nastier critters, you may consider a 35 whelen, or a 375H&H.