Depends on what you plan on doing during the SHTF scenario. First, forget fighting off the massive rampaging hordes in human wave attacks. They don't happen, and if they do you aren't stopping them without something crew-served. Second, are you holing up or moving out? If you are holing up weight isn't a big deal, if you are moving out then it can become important. Third, urban or rural? The up-close and personal nature of the urban combat is very different from the wide-open spaces. Finally, are you going to be a rabbit or a tiger? Do you run and hide, or fight it out?
Personally I think most folks are better served with the shotgun when you deal with reality, but the battle rifle makes for a more macho image in the mind.