.300 Win Mag (I am assuming this is the 300 mag your talking about) is an excellent antelope round. I would always take the .300 over the .243 for this type of hunting unless you shoot your antelope at pretty close range. Out here in Idaho, most antelope kills are a few hundred yards out. The .243 starts to drop off pretty fast out past 300 yards. I would concur with the previous advice concerning using 180 grain slugs over 150 grain slugs. They carry the energy further, and if you just happen to get a shorter range shot, they will definately not be moving along as fast as the 150's.