When it comes to bear guarding, a one size fits all approach to the gun you'll carry is a mistake IMHO. A shotgun has the advantage of firing less lethal ammunition such as cracker shells and rubber slugs, although personally I'm opposed to the use of both lethal and less lethal loads in the same gun. If you're close to town, one of the small cottage subdivisions or the CNSC, a rifle is not suitable due to concerns of overpenetration. If you are guarding a construction site where workers are spread out over a wide area, a shotgun or a low velocity iron sighted rifle renders you a spectator rather than a participant. Likewise guarding a large tour group invariably leads to people being strung out over a long stretch of ground; some participants are jocks and some are geriatrics. Its up to you to rein in the athletes so they don't get too far ahead of the flower sniffers, but like herding cats, the line tends to get strung out.
Since problem bears are typically sub-adults, big hard kicking rifles aren't essential, even though they might make us feel better. The trouble is you never really know what scenario you'll face. If you're on foot all day, you have to make the assessment of where you'll be working and gear up accordingly. If you are moving site to site by vehicle, you can literally have a golf bag full of guns. For hunting any North American game, I think a 6.5 that can drive a 140 gr bullet 2400 fps is a reasonable minimum, but for protection work a .30/06 is a practical minimum. Certainly less gun can work in the hands of a cool shot, but when lives are at stake and the range is short, its fair to say that "cool" is a subjective measure. Many swear by a 12 ga slug gun for bear protection. If you can shoot slugs from 12 ga competently, then you can shoot a .300 magnum competently, since its recoil energy is similar and rifle stocks tend to be more ergonomic resulting in less felt recoil. Should you find yourself in a situation where the range exceeds what you can manage with a 12 ga, a .300 magnum comes into its own.
These days I tend to carry a 12 ga more frequently than anything else. My bear guarding gigs tend to be with individuals or small groups, so reaching out isn't my immediate concern. Its also the gun I most frequently carry for personal protection. Now that's its cold though, a rifle will be my primary choice, except close to town. The reason is that I've had too many incidences of shotshells splitting when fired in the cold, and I can imagine that a split shell could tie up the gun, preventing a follow-up shot.