I use a 12ga primarily because I got the combo and wanted the 12 for pheasants. My son's combo is an 870 in 20ga, and all his deer have been one shot drops. (He's pretty good on the pheasants also.) If I were to get a dedicated deer gun, I would not hesitate to get the 870 cantilever slug gun in 20ga that you mentioned. With the new sabots, the energy difference is surprisingly small, especially past about 75 yds. Actuall, for a dedicated slug gun, I would probably go with the H&R/NEF Ultra Slug Hunter. Love that gun in the laminated thumbhole (about $325), less for the other stocks.

http://www.hr1871.com/Firearms/Shotguns/ultraSlug.asp