I don't think that the jury is still out on the .300 wsm.. the verdict is in and this caliber is here to stay. I have a Sako chambered in this caliber and love it, although I don't really think I would love the regular win mag any less if I had that.
The bargain ammo (blue box federal etc) will be cheaper with the win mag, but the premium stuff is very similar in price. Given that you are looking to use this as a hunting rifle, ammo cost wouldn't be an issue.
If you want to get into reloading, then the regular win mag offers you greater flexibility to use heavier bullets and the larger case capacity will almost always give a slight performance edge, but again, not enough to care about unless you want to go 200 grains and over in bullet weight (if you do, skip straight to the weatherby anyways).
At the end of the day they are both awesome calibers and it comes down to personal preference.. the wsm does offer some real advantages in terms of the short action, lighter weight and the ability to achieve good velocity out of a shorter barrel. The design, being newer, also has the theoretical edge in accuracy and efficiency, although let's be real here.. anyone asking questions about which one is better is not in a position to be able to notice any difference (myself included).