Re: the magnums:short/long
I think the short mags give you two advantages.
First, efficiency. With a short mag they will perform as well as a bigger magnum with less powder.
Second, a tendency toward better accuracy. Pretty much any cartridge can be made accurate, but the short mags tend to be inherently more accurate. Whether it is because of shorter, stiffer actions, the overall efficiency causes less of a velocity spread or that a shorter case can tend to deal with headspace better, it is all debatable.
The issue with larger bullets is due to short actions. To get them to fit in the magazine you end up seating them very deep. If you choose to single feed and seat to the lands with most factory barrels you can shoot big bullets.