My choice would be the 300 WSM
. Not because I have one, but simply because of the availability of projectiles on the market. Yes, the 7mm and 270 are quite abundant, but the "go to round" for decades in the law enforcement and military families was the 30 cal. That alone was my deciding factor in my purchase of the 300 WSM.
Yes, I could have picked a different caliber that wasn't a WSM, but my thought when comparing the 300 mag and 300 WSM was on the financial side. Yes, the WSM is a more expensive caliber to purchase... but when reloading, you use less powder and get NEAR the same speed and trajectory.
I know... how much powder does one really save... not much. But lets just say for a moment that you're a shooting nut like I am. 50 rounds on a weekend... 52 weekends a year. That's 2600 shots a year! Now, lets say you save 3 grns per shot... that's 7800 grains (over 1 lb) of powder a year! Now, you keep the rifle for five years... that's well into the realm of 6 lbs of powder saved over some 13000 shots! 6 lbs of powder goes for say...32 bucks a lb, that around 190 bucks. BEER MONEY!
You see where I'm going with this. In the end, the choice is yours and yours alone.