Generally, I prefer .30 caliber, (big ones), but of the 2 choices you have, I would start with the MK X in 7 Rem Mag. High B/C bullets around 160 gr weights. Don't know much about the .300 WSM, but my idea of a L/R .30 cal is a .30 x .378 Wby with 200 gr bullets.
If you don't intend to rebarrel and restock, etc, just pick whichever one of the two you have that is the most accurate and work with it. A 1 moa rifle is hardly adequate for long range shooting. A 1/2 moa rifle is good, and is better than most shooters, in the field. Find the maximum range that the rifle can shoot 1 moa or under, and don't use it past that distance.
Keep good records, pay attention to and record conditions, and get good at least out to 600 yds. Every chance you get, shoot 1000 yds. 1 moa at 600 yds is 6". That's enough error to wound an animal. At 600 yds, the wind will push you around. At 1000 yds, it'll send you home ****ed. After 45 yrs, I still suck at doping wind.
The most important thing you will learn by shooting a lot at long range is when NOT to take the shot. When you punt one, do it at the range. A wounded paper target won't run off and die a slow death.
I apologize for getting preachy, but I've learned that there is no shortcut to good long range shooting and ethical long range hunting.
Good luck to you, Tom