I can tell you that the H&R USH 12 gauge does not kick nearly as much as a Mossberg or Remington pump. The weight simply soaks up more of the recoil.
12 or 20, once sighted in you don't have much of a need to shoot much anyway. Maybe a bow before season to make sure the zero is still there and then a slug per deer.
The 20 is a great slug gun but I shoulder a 12 because I like to deliver alot of lead.