I see nothing wrong with the .300 for both hunts.....out in the sage U might need to make a long shot on the mulie, and the .243 will be lacking.......If U just want to shoot a animal and aren't a meat hunter I'd say go with a .257 weatherby, any shot closer than about 400 yds ya gonna ruin alot of meat......... but beyond that it would be great, good caliber for just about anything, has killed elephant and cape buff!
If ya really good with yer .300 why go with anything else!!??!!