Personaly I would go with the 300win. Nothing wrong with the WSM, I've just shot the crap out of the 300win and think its about as perfect a medium range round can get. You can buy factory mach ammo for it, plus the slight velocity gain will help with hevier bullets if you really get past 1,000yds. If you want a Lapua, you'll have to go with a LA anyways. Now having said that the WSM's I have seen shot, did very well, and as long as you stay with 190gr bullets or less, you really dont lose any velocity over its big brother.
From having owned and shot most systems out there I believe if someone can handle the recoil about the best bet for an all around 0-900+yds gun for somebody starting out or that doesnt want to go custom, is a Savage 110FP in 300winchester. Haveing set up quite a few for buddies, its been my expierence, after makeing sure that the stock doesnt touch the barrel, with a good scope they'll AVERAGE 3/4 to 1 MOA at 400yds with Federal or Blackhills 190grSMK match ammo, and under 3/4 minutes with handloaded Amax's. The one I have now averages 2 1/2 to 3in at 400 and 4-5 inches at 600 with Amax's.