12 pump may be quite good choice, but I personally take sxs or O/U if have to take a shotgun. I have used 870 rem as my primary shotgun for 11 years now and I thought that have learned something about it. Pump action could be very fast but I disagree what Daryl_s said that pump is much faster than autoloader. Autoloader is allways faster than pump but pump is allways more reliable.
For wounded bear I might use 9,3x74R/12g combigun and if there is larger bullet caliber available I prefer that. For 9,3 I choose 320gr Woodleigh and for 12g 8,6mm buckshot.