Don't get all wrapped up in excessively high magnification at the expense of low end magnification. 10 - 12 power is more than enough magnification for big game hunting out to several hundred yards. I personally wouldn't want to go over 4X magnification on the low end unless it were on a long range specific rifle.

With a 50mm objective lens you are going to get in increasingly darker image when you get above 10X magnification. Just because you can crank it up to 18 or 20 times magnification doesn't mean you are going to be able to see what you are looking at.

Leave the high magnification scopes for the specialized long range and varmint rifles.