I agree with Boss Hoss. If you've got your eye on HUGE case capacity and the 300 grain MK, do the Lapua mag improved. It gets rid of all that nasty case taper which only makes you trim and FL size 'till you hurt. I have never been a fan of the standard Lapua mag. Looks too much like a 22-250 only taller and bigger.
The 338 edge or the 338 ultra mag will nip at the heels of the Lapua mag (and in some cases even beat it) in the velocity category and do it with a much straighter case and a sharper shoulder angle. Sharper shoulder angles, by the way, have been proven to be more efficient especially where heavier than normal bullets are being used.
Also, you can utilize a standard magnum bolt face with the Ultras. Once you have the bolt made for the Lapua, you are stuck with it forever and that is a bad thing if you ever want to try something else.
REm Brass isn't as good, but you can just buy 2 more bags of it than you think you will need and sort them out. Piece of cake.
Here is the Lapua standard and the Lapua improved side by side: