RE: 45-70 or 450 marlin?
If you load your ammo you are far better off with the 45/70. It has more case capasity and when loader to the same pressures as the 450 it blows it into the weeds!!
You can push the 250 grain Barnes Expander to 2650 and still stay in the pressure limits of the Marlin. By the way all NEW Marlins have the same pressure limits, (at least the big guns, 336 sive and up) For the most part you do not want to exceed 36,000 C.U.P. acording to the book.