I must agree with Gforce, if you dont have a 30-06 a 7mm Rem Mag does real good. Although I have a 270, I still see that it just doesnt have that same mmmph that you get with the 30-06, especially since a perfect broadside doesnt always present itself. Just recently I had a hunt where the 270 wsm (close enough to 270 win) just didnt have the extra penetration for those quatering away shots, it did the job, but not efficiently enough. I think if it were a 30-06 it would have been a different story...
Best advise is take what you are most comfortable with (270 to .30 cal are all great for plains game) Enjoy your first hunt!!!!