For the price they are asking just buy both.. :wink:
Seriously, the 20GA does kick less but it is not more or less accurate than the 12GA...It all comes down to personal preference. A plus for the 12 is that the 12GA sabot slugs are more plentiful than the 20GA sabots so you should have a greater selection. Of course this may only be regional and your results may vary. This is just my experience in Western NY State. I have a 12GA now but I have considered picking up a 20 USH Deluxe for whitetail season...Oh well, I have too many other things that I need before that so I will probably just keep writing that I plan on getting one some day.
Good luck.