It all depends on what you are intending to do with the gun and how much recoil you are able to handle. For bear hunting in Alaska for grizzly, most guides will want a weapon that has power and distance. In that case, the magnums are the way to go starting at .338 magnum levels and up.
For close quarter brush protection, the 45-70 is the most popular loaded with +P loads such as the 540 gr Garrett Hammerhead that will get attention at both ends of the barrel. Recoil energy for many is over 40 ft-pounds. For practice, you can use standard loads that are very reasonable.
I chose the .444 for large bore protection and reduced recoil especially by adding some weight and concentrating on recoil pads. The way I have mine set up, it is like a 20 ga in recoil. The .444 is a VERY accurate gun as well. With my Skinner sights, I can put a 1.5 inch group at 100 yards.
The difficulty in the debate between .444 and 45-70 is the lack of popularity of the .444 giving few opportunities in real to show it is a real contender. A couple months ago, a hunter in Canada stopped a large grizzly in full charge with two well placed shots to save his life. I posted previously on TFL that encounter:
http://thefiringline.com/forums/showthread.php?t=467334
As much as I love my .444, if I was younger, I would indeed consider the 45-70 if you are looking at getting the power of +P rounds and 500 gr bullets. In my .444, I use the Buffalo Bore 335 gr bullets at 2025 fps. I feel comfortable that it will do what I want it to do as a defensive woods gun God forbid I ever needed it.
Bottom line, it all depends!