It all depends what you will use it for. 308 is plenty available for much lower prices than any other 30 cal cartridge (except 7.62x39 of course). Have you looked at the cost of 300WM? Not many people are using it, in fact I have 100 rounds or so here which I got for free with some other ammo, zero interest when I wanted to sell it. Most people wanted it for free. I'd rather keep it.
Most people are going from a 308 to a 30-06 before considering a 300WM, but then there are those who never get enough. Why not a 338 WM? In layman's terms, the 30-06 had more powder than a 308, the 300 WM has more than an 30-06, the 338WM has more than a 300WM with the 338 pushing a .338 bullet. The higher powder charge is intended for longer distances. The drawback is price and for the user recoil. I do not like heavy recoil rifles and I don't need them, sure, you can put a muzzle brake on or a butt pad, for me, all those cartridges are just not useful for what I am doing.