Use the gun you have supreme confidence in. I say it's more imortant to have confidense as well as proper bullet selection than caliber. I shoot everything with the .300 Win Mag. A guy I work with uses a .338 for everything. Sounds huge for lopes, eh? Proper bullet selection and there is no more damage than a much smaller caliber. I've seen it both as a hunter and guide.
Best advice is practicing 200 yard- 300 yard- or more shots and know your balistics.