RE: Regret using 20ga for Whitetails?
the H&R will probably be the most accrate. Those saddle mounts have a bad reputation for not holding accuracy since theyre not connected directly to the barrel, and are bolted through the reciever.
I'm not a big berson at all, and even the 12ga H&R i handled the other day wasnt THAT heavy with the scope mounted, and the 20ga is a pound lighter.
As far as ammo, i have been doing some digging, since i am a shotgun only hunter (and will be getting a H&R in 20ga soon) and it looks like the 3" Barnes expander slugs from federal are the best bet for long range performance, holding almost 1000 ft lbs out to 200yds in a 20ga :O
Ive knocked them over with my 20ga and copper solid slugs out to 120yds, so the 20 will be all the gun you'd ever need.
Mat