Honestly, either one will work fine. Technically, the 243 will hold about 2-4 grains more powder. You will gain about 50 fps, maybe 100 fps. They are both very similar though. The CM has the optimal 30 degree shoulder for accuracy with ease of feeding from a mag, so it's a toss up. Pick one, work up a good load, and practice with it, and I doubt you could tell the difference.
"I don't forget my bullets at home"- That's funny!!!