Pro's for Lapua: faster (300 fps with 250 gr bullets) slightly better trajectory.
Con's more recoil, ammo is special order at most retailers.
Pro's for win mag: less recoil, less expensive =more shooting, ammo available most places.
con's: less velocity, but not much.
Elk can't read ballistics charts, so they probably don't care which one you get (they will be just as dead with either)
The difference in velocity translates to roughly 13 inches more drop at 500 yards for the win mag. The Lapua has 37 inches drop, the win mag has 50. You will be turning dials to hit at that range with either.
If you can afford the Lapua get it. Don't feel like you would be short changed by the win mag if that is what you can afford to shoot. There is not enough difference to matter until after 1000 yards. Then I say Lapua all the way!
You can't fix stupid....however ignorance can be cured through education!