Wanting to get my dad a spotting for Christmas, we have an old Nikon that we busted on a trip out West several years ago. Looking at the Vortex Diamondback series, any input on whether to get a 16-48 or 20-60. For around Kentucky he will mainly use it on our back porch to look out in the field for deer and turkeys which is around 600 yards at the furthest point and I might borrow it for shooting at a 1000 yard range. Thanks for the input.