Why the insistence on a variable scope? For 100 yard shooting with a .308, a fixed 2.5X or 3X scope seems more than adequate for a non-sniper rifle. It'll also work fine if you get a chance to work at longer range and let the rifle "stretch its legs."
I have a .308 bolt gun with a fixed power 2.5X Leupold (scout style, mounted forward). I am very satisfied with it. For a .308 battle rifle, I'd get something like a 3X ACOG or similar and call it good.
Of the varibles you mentioned, I'd go with the lower power. Anything over 4-5X seems like overkill with a .308.