.30-30 is more'n enough for black bear within 150 yards with a 170 grain bullet. I expect it's killed more deer and black bear than about any other caliber. The rifles are accurate, let no one tell you different. A good lyman or williams receiver ghost ring sight really helps. My friend has a pre-64 with a ghost ring receiver aperture sight and I can put 'em into 2" at 100 yards with it, don't get a whole lot better. Yeah, I don't have, nor have I ever had a 94 or Marlin lever gun, but I won't sell the .30-30 short. I did have a Savage M340 bolt gun once and shoot it in a 12 inch contender barrel now. I load Nosler ballistic tips for deer and Barnes 140 grain X bullets for heavier game like hogs and it has anchored everything I've shot so far, all inside 100 yards. Out of that gun, with those bullets, it's good to 200 yards on medium game from that pistol barrel.
The .300 mag is excessive, but you don't have to shoot full power and you can choose a heavy bullet that won't tear up so much meat on deer and will still kill bear. Heck, it'll kill BROWN bear, much less blacks. Black bear ain't that tough to kill, thin skinned really, not that I've ever shot one, though, so don't listen to me.
But, .30-30, .257 Roberts, 6mm Remington, .308 Winchester, I've seen all these calibers recommended as good black bear medicine. If a .44 mag can stop one, I KNOW the .30-30 will. Nobody seems to argue the .44's worth on black bear, even in short barrel handguns. Well, a .30-30 with a 170 grain flat nose has a better ballistic coefficient and is starting out with about 1800 ft lbs of energy, well above a revolver fired .44. Those big flat nose bullets have a better SD and open very reliably on impact. That is what has made the .30-30 a great killer of medium game for 112 years.
So now, there's one determining factor on which I'd take, RANGE. If you think you might be shootin' out at 300 yards, forget the .30-30. I haven't looked at the new Hornady extended range stuff, supposed to be good, but 300 yards is WAY out there for a .30-30, on the edge of even a good boat tail bullet load fired out of a rifle. The .300 would be the better choice over 200 yards IMHO.