the two caliber choices listed are at least marginally redundant in the type of game they would be used for,
I personally don,t see the real benefit of having both in your gun safe.
as to the choice, between the two Id be marginally, more inclined to go with the 300wm , mostly because you have a 243 already.
but be aware that the difference in selecting the 300wm, potentially comes with the advantage of being able to use heavier projectiles,
in the 180-220 grain range, and the down side is a bit more recoil as a result
the difference from what Ive seen in guys using both is rather marginal, both get the job done if the guy using either can select the correct ammo and shoot,accurately,
its kind of a do you prefer blondes or brunettes, ford or chevy deal.. you can,t really go wrong with either choice.
compare the load data and youll see the differences
300wm
http://www.handloads.com/loaddata/default.asp?Caliber=300%20Winchester%20Magnum&Weig ht=All&type=rifle&Order=Powder&Source=
7mm mag
http://www.handloads.com/loaddata/default.asp?Caliber=7%20mm%20Remington%20Magnum&We ight=All&type=rifle&Order=Powder&Source=