If your looking for a rifle that you can do alot of target work with I would go with the .300 WSM
. The .300 RUM is just to hard on barrels to see alot of rounds. The other consideration is the weight of the rifle, generally most guys prefer a heavier rifle when it comes to the ultra mags, and you said you want kin of a "cross -over" so the lighter recoiling .300 WSM gets my vote.
If you plan on shooting it very little then the Rum will get you out to 700-800 yards a bit easier. The WSM can also use a shorter barrel. I know you said you dont want a .338. but I feel that the .338 RUM would meet your needs well too.