If this rifle is for hunting only then a 3-9X is all the scope you will need on top of a .308. Honestly on a big game rifle I feel that any magnification over a maximum of about 12 power is highly unnecessary and 12 power is really pushing it. Also IMO 4.5 power on the bottom end is too high on a hunting rifle.

Unless you are wanting to shoot at very small (prarie dogs) targets at very long ranges (400 yards+) then get the 3-9X

I know you said don't say it but it really does depend on the gun, the ammo, and the person behind the trigger. A good shooter will be able to shoot to several hundred yards with a 9 power scope and for the average shooter it is plenty enough magnification for 400 - 500 yard shots.

My 300 wsm has a Burris 3-9X, My .338 Ultra has a Burris 3-9X, My .270 has a Bushnell 3-9X, and all of my other high power rifles are all wearing 3-9X scopes except for one wearing a Simmons 2.8-10X.