I'm thinking we are needing some clarification on application here. I took walkabout to mean non-hunting hiking. This would mean any shooting would be in a toe to toe scenario, with something very bitey. Down range energy and trajectory are thus meaningless, frontal diameter, muzzle energy and point blank penetration mean everything. Speed of action could also mean winning or losing this showdown, so if this be the intended purpose the 45-70 is far and away the better choice of the 2 choices given by the OP. There is something about that large flat meplat of the 45-70 that tends to take the fight and bite out of bears very quickly.
And let me say, before somebody chimes in with "it doesn't matter which, if you place the first shot well", this scenario doesn't usually allow for accurate first shot placement. You may well have to shoot at a rapidly approaching target through willows, buckbrush or thick stands of small poplar as this is generally where these encounters take place. You are almost never in danger out in the open with lots of visibility, you have to inadvertantly invade the bears personal space as a rule before there is a confrontation that has to be settled with a loud bang or two.
Having been in this position a time or 2 (never had to shoot as they were bluffing), I would want to be carrying the 45-70 with handloaded 400-500 gn hard cast bullets with a large flat meplat, or a 500 gn round nose jacketed intended for the .458.
Allow me to paint you a picture, and then you can decide which rifle you think is more suited.
You are climbing up a mountain to photograph some Dall sheep up in an open basin. You're fighting your way across a wet bench thick with 8 foot high red willows when from about 20 yds downwind you hear a WOOFFF. Then a growl and then a sound like someone hitting two fir 2X4s together at about 1 second intervals and it is getting closer very fast. You can hear the willows moving and the dead ones snapping but can't quite get a fix on it. You realize you can't see 5 yds and there is absolutely nowhere to go and not a tree within a mile. You scream a few profanities as loud as you can at the incoming noise and fire a shot in the general direction of the sound and reload. You're staring intently into the willows where the sound had been approaching, you can see the whisps of gunsmoke amongst the willows and smell it in the damp heavy air........SILENCE!!!
This has all taken place in 4 seconds.
Which rifle and caliber would you rather be holding at this point?