Believe it or not, both will shoot about the same trajectory out to 600 yards. The 300 Win will begin to shine beyond that range with flatter trajectory (up to 10 MOA less at 1000 yards) and less wind drift. I've shot a lot of 30 cal mags over the last seven years, all of them a lot bigger than the Win Mag. After all that, I came back to it. Forget all the crap you hear about the neck being too short or the case being under capacity, etc., etc. In 1000 yard benchrest, the 300 Win Mag is one of the dominant cartridges. With today's powders and bullets, it may well be the most efficient case on the market, 300WSM notwithstanding. If you really want to reach out, try a 240 grain Sierra at 2800 fps; this begins to rival the 338 Lapua in terms of performance. As far as cost goes, if you reload, its not that much more than the .308, but you will pay a price in recoil- no free lunches. If you don't reload, well, the .308 Fed Match is 40% cheaper than the Fed Match 300 Win. Like Garand said, important consideration. As far as accuracy, the Win Mag will shoot every bit as well as the .308, even better at long ranges.-
Its nice to say "well, I'm not going to shoot beyond 600 yards". That's like saying "well, I'll only have sex once." I recommend being prepared.
Squeeze'em crisp.