Scopes with thin cross hairs work best for me as I can disect the dog better in the sight picture, and as I see the bullet impact, I can adjust clicks on the scope. Also, as the distance increases, your cross hair covers more of the dog, this is a critical issue. You do not want your cross hair being larger than the dog.
I shoot 6.5x20, 6x24, and 8x32s. I shoot a lot from 24x-28x.
If you use a mirage shield on your barrel, barrel heat will not be a factor, these are simply made with cardboard and double stick velcro. Sun shades make your scope darker, use them only if necessary.
One of the best scopes we ever used was a Baush and Lomb(bushnell) 12x32 that has a fine cross hair, small turrents, and repeatable click adjustments.