For 180 gr & lighter bullets is go WSM cause they seem to out perform the WM for less $ in powder. For larger bullets is go WM due to the larger case capasity for seating long bullets & having room enough for the powder to push em.
Your splitting hairs in actual performance, & it seems to me it is more of a preferance thing.
I love loading for my .270WSM, but hate loading for my .300Wby. The WSM is a beltless case (the parent case being the .404) I prefer these. The Wby & Win cases are belted simply because Holland & Holland used the belt for headspacing, & the craze of the "belted Magnum" was in full swing. I personally don't like the belt, & its just 1 more hoop to jump thru while setting up your loading dies. Not really harder, its just the fact that the belt is not needed, & therefore a waste of my time, & it irks me for some unknown reason. I like simplicity I guess. But I wouldn't hesitate on buying either rifle. Both are exelent. For me it would be WSM cause I even load my Wby with 180's. But you may want to launch heavier higher b.c. bullets so I guess the advantage would go to the WM for that.
It's a personal preferance thing I.M.O. because they are very close compared side by side.