My late friend Stu Carty (also a friend and business partner of RinB, and a real rifle loony) took five eland during his several safaris--four with the .30-06 and one with the .375 H&H. He said all the .30-06'd eland died quicker....
Just thinking that the .338 will shoot a bit flatter for any 300 yard shot...I don't dial.
I m guessing you're talking about the .338 Winchester Magnum, which I have a bunch of experience with, both in Africa and North America (along with a little in Europe).
It's basically its a big .30-06. With bullets of the same approximate sectional density (and hence B.C. is shapped similarly) it gets basically the same velocities--whether with 165s in the .30-06 and 200s in the .338, 180s/225s or 200s/250s.
