I'll echo Ceejack when he mentions the need for a quality scope! In most cases, you'll end up paying more for the scope than the rifle. I think if you are strictly sticking with deer, I'd opt for the 270WSM it will perform well for you and won't beat you to death. Otherwise, I'd probably tune up your 300 Win Mag and start shooting it. Consider an accurizing job through your gunsmith if you can't shoot 1 inch groups at 100 yards and take up reloading if you haven't already. By loading up your own ammo, you can really tighten those groups up. If you're wanting to buy a new gun strictly for long range hunting, I would agree with your decision to do so. I'm new myself and bought a 7mm Ultra Mag in a Remington 700 Sendero that should fill the bill for most of my shooting but I would recommend that you check out a custom rifle
if you plan on getting super serious about it. Research and decide wisely.