Between the two it would be a toss up to me as I have never owned either, but the 7mag shows a tad better ballistics in some departments.
As far as the scope deal goes it doesnt matter what power you set the scope on, the cross hairs never change so it cant be off, they are just as fixed as fixed sights, only your eye would be focusing, does that make sense ? I do notice the the higher end scopes operate better in low light conditions and tend not to fog or condensate outside the lens and look clearer when they are set at high power such as 14,16 or 20 power. I have a bushnell banter 6x18x50mm and it doesnt impress me like my old Leopold did, it actually pissed me of a time or two.