On my biggame setups the low power is 2.5-3.5 and high 10, I have never seen the need for higher than 10 power for a biggame rifle. Most of the time my scopes sit around the mid point - 6 power but on occasions where shots are going to be tight or close I will lower. I like the ability to quickly acquire the target and not have a blob type image in the scope.

Objective size is personal but don't buy a larger objective in a cheaper model and expect great gains. By far you are much better to buy a scope with better lens coatings in a smaller objective!