All kidding aside, the difference between these two rounds couldn't be more plain to see. When I started thinking about getting back into shooting after a number of years off, my fondest dream was to have a 30-378 Weatherby with a huge (3-15x or 3-18x) Swarovski scope so that I could be the total master of the earth (at least within eye-sight of my position) just the way they taught me in the USMC. I would be lord and master, life taker and heart breaker...........No one would dare to challenge me and my 180 grain X-bullets at 3600 fps with only 19" of drop at 500 yards, NO ONE!!! HAHAHAAHAH!
After going out and shooting my grandfathers rifle (which I admit), I considered to be 'second rate', I have changed my opinion. There are some marvels of engineering that transcend time, (in my grandfathers case) the 270 Winchester, and in your case the 45/70 Govt.
The 45/70 has been here for all these years because it works. This new 338 federal might be gone next week, and that would leave you loading what would be considered a 'wildcat' round. Previous poster RCModel was right when he said that you could get 45/70 in moose-dung Alaska. They have them now, they'll have them next year, and they'll have them in 40 years time too.
What I'm saying is to invest your hard earned money in things that are truly great, not in things that just showed up last week.