The answer is different for nearly everyone, as everyone's circumstances cover a broad range of possibilities. Are you going far enough north to get into polar bear range? Are you engaging in an activity that might cause you to interact with bears more than simple chance encounters? What is your shooting experience? What is your back country experience? Are you physically fit? Will the bulk of your time be spent on the ground or in a canoe? Will you travel by light plane or helicopter? What is your budget?
Then again the question can be easily answered by saying carry the most powerful rifle you can shoot well, shoot quickly, and carry all day without undue fatigue, but your answers to the questions above will have a definite influence on your final choice. For ease of carrying, nothing beats a traditional lever action rifle, but they are more prone to damage than a handy bolt gun in a fiberglass stock. Still the M-94 .30/30 has served those making a living in the back country as well as recreational hunters for generations. It carries easy, doesn't kick much, and is quick to bring to bear. IMHO, the factory sights on these rifles should be exchanged for a ghost ring and rectangular post. Stay away from the Mare's Leg, unless you intend to put a standard buttstock on it. The point of shooting is hitting, and without a buttstock practical marksmanship is far more challenging than you will appreciate. Moving up the power scale, a .308 or a .30/06 is a good choice, if as I suspect black bears are your primary concern, and about the sensible minimum for polar bears or grizzlies, but not if you are unfamiliar with the rifle you purchase, or if you consider these hard kickers. If you've cut your teeth on bolt actions, and can handle a 9.3X62 or a .375 of some flavor, you could do worse, but then again, if that was the case, you wouldn't have asked the question. If you have the means, you could purchase a dedicated bear rifle, and the internet is full of ideas for custom dangerous game rifles from builders like Brockman and Wild West Guns, and similar rifles can be built by any number of talented Canadian Gun Smiths who will happily build you a $5000+ rifle. If that's not the case, don't get fooled into going the bargain basement route for what is essentially a piece of life saving equipment. You get what you pay for.
The shotgun you have and are familiar with, might trump the rifle you don't. These guns though tend to be bulky compared to rifles, and you would be unable to carry as much ammunition per weight as you would rifle ammo, even if you chose a .45/70 or a .458. The shotgun though has the option of using less lethal ammo, although you should be aware that using this ammo can cause partial barrel obstructions, so ensure your bore is clear before firing a conventional shell. Doesn't that sound like fun if the bear you've attempted to drive away continues to approach, which is one reason why I prefer to have both a scare gun as well as a kill gun. Another reason is that in the spur of the moment, you might shoot a slug when you intend to shoot a rubber slug or shoot a cracker shell when you needed a slug. Cracker shells by the way have a nasty habit of starting fires if the country is dry, so keep that in mind. Should your travel include a helicopter or light plane, a gun that has a collapsible stock and short barrel is a benefit, and a soft case might provide some protection for it.
Regardless of the gun you carry, ensure before you go that its in good mechanical condition. Be sure the sights won't get lost on a dark background particularly if you are going into the thick stuff, and black bears are your concern. Ensure the gun is sighted for the ammo you choose, and that it hits point of aim at close range. Ensure the ammo you choose will cycle through the action without any hic-cups. Carry a sectioned steel cleaning rod with a jag and patches to clear any barrel obstructions. If you don't intend you use less lethal ammo, taping the muzzle of either a rifle or shotgun is beneficial.
