If you do not mind the weight, the USH is the ticket. It is more accurate, and you would have a dedicated gun for this purpose and more cost effective. If you already have the 870, buy the slug barrel, it is much lighter, you have the ability for quick follow up shots, and it will be as accurate as you can shoot it.
I have a buddy that has a USH and loves it. I have an 870 with a slug barrel and love it. I already had an 870 for pheasent, so all I needed was a slug barrel. My buddy did not have a pump shotgun, and he did not mind the extra weight. I hunt much deeper in the woods, and really do not want to lug around the extra weight.