Probably too late, but with todays 1800/1900/2000 FPS slugs, the 20 ga is all you will ever need to kill whitetail. You are basically shooting the same bullets as the modern muzzleloaders do, saboted 45 caliber bullet. The shotgun is shooting them faster than the muzzleloaders do with 100 grain charges, and nearly as fast as 150 grain charges. The bullet don't know what kind of gun it is shot from, as long as it is accurate and you are capable 150 yard shots are not a problem. Most of these bullets have enough energy way beyond the 200 yard mark, so 150 is no problem as long as the shooter is up to it.
If you are going for bigger game than whitetails, you may want to consider the 12ga. The 12 ga is very heavy, but the slugs are bigger and have more thump.
I have shot deer out to 135 with the 20ga, and they drop just like they do with a 12ga.
Most of the new 1800+ FPS slugs use a 150 yard ZERO on their ballistic charts, average about 2"-3" high at 50 and 100. Anything out to about 160 yards is just aim and shoot, takes out a lot of the guesswork.