It would depend on the hunting conditions. Three very respected gun editors chose three very different "ideal" cartridges for elk. Elmer Keith, an Idaho rancher and "dean of the gun writers" in his day claimed the 45-70 was ideal. But he did most of his elk hunting while providing a railroad construction crew with meat in the early 1900's. Lots of elk, few hunters. His rival, Jack O'Conner, claimed the 270 Win was perfect. However, he lived in northern AZ and had the time to pick and chose his shots. More recently Jim Carmichel (If you don't own his Book of the Rifle, get a copy.) picked the 338WM because he didn't have the luxury of time and thought the 338 would kill an elk from any angle, even at a distance.
Your best bet is contacting your guide. He'll likely tell you that a 30-06 or 270 works best if you are comfortable with the recoil and it's an old familiar rifle.
