I kinda like the idea of the 405. Not that any animal is really going to be able to tell the difference. Don't worry about finding quality bullets for it, Barnes, Woodleigh, Swift, Hawk and others all make high quality 411 diameter bullets that will work in the 405. Shoot Woodleigh and Barnes makes 400 grainers that have a greater sectional density (better penetration if comparing jacketed bullets) than the 405 grain offerings in the 45/70. Plus, if you get the Winchester 1895, you can shoot spitzer shaped bullets. Swift makes their A-Frame in a 350 grain .411 and should be a dynamite combination.
If you are going to use factory ammo, the 45/70 might be a better bet, though with Ruger introducing the 405 in the No.1 this year, more manufacturers may consider offering that load. Also, with H&H introducing their new 400 H&H (using the .411 bore), yet more bullets may be available. If you want to shoot hardcast slugs, the 45/70 is definitely the way to go. But if you want to use jacketed bullets and are a handloader, the Winchester 1895 and 405 Winchester is something that really deserves looking at.
Personally, I'd go with the 405.