Pick your poison, PLENTY of elk are killed each year with both the 270 and the 308. Quality 150s in the 270, and 180s in the 308 and if you get the shot you are ready for it.

With 180s the terminal performance of the same 180 in either the 30/06 or the 308 is identical. NO ONE would question the 06s capability, it's killed more elk than anyother caliber. (I'd stick with the Barnes X but any quality, heavy penetrating bullet is a good start for a ol screamer!)

Good luck,
HL