Usually the non mags are more accurate and in a 300gr either will likely pass through on a broadside shot. I prefer the non mags mainly because they obturate better. In other words they fatten up in the sabot as they go from 0-XXXXfps. The mags have thicker jackets and don't obturate as well.

This isnt set in stone since some people have very good luck with mags but they are harder to expand on impact as the fps drops.