Smitty,
Any of the catridges you mentioned, for the applications that you stated, should be fine.
A 7 Rem Mag, with stout bullets and, of course, proper placement, will work on any elk. A 300 Win Mag is more "effective," but comes at the price of higher recoil which you may, or may not, handle. The newer 300 WSM would be great, but, unless you are a handloader, bullet selections are not anywhere remotely near that of the 7mag and 300 Win. mag. Also, you have a great deal many more choices in rifles with the 7mag and 300 Win. Mag than you do with any of the WSMs.
Another consideration? If you happen to go elk hunting (or I guess hunting for any other game animal in a relatively remote area for that matter) and you happen to have either forgotten your WSM ammo, or lost it, or had it fall over a cliff into a deep fast running river or something, chances are that finding WSM ammo in a local store would be very difficult compared to buying good ole 7Mag and 300 Win. Mag. ammo.
Zachary