General rule of thumb, buy a .308win. Enough said on that.

Oh, you asked why: It's capable of anything and everything that the .30-06 is, which the .30-06 is truly capable of virtually everything that the .300 mags are capable of, so although no one really ever says it that way, the .308win is just as field capable as any of the .300 mags (which has also been my experience, not just an assumption), BUT, it lacks in one very important aspect to all of these cartridges....

RECOIL, the .308 is a joy to shoot, while putting more than a few rounds out of the "stronger" 30cals can get pretty painful pretty quickly.

Bottom line, the .308 is proven on every north american game species, and in anyone's hands is perfectly capable of taking any of these species except maybe the three great bear species...which are typically reserved for bigger calibers, not just bigger cartridges.