Steve,
Almost all factory ammo for the .45-70 is loaded light in deference to all the old 19th Century rifles that are still around but were designed for blackpowder pressures. These old guns are good for maybe 18,000 psi or so, go much above that and you'd be picking pieces of metal from your forehead. The .444 Marlin is a new creation, it's factory loaded in the 50,000 psi range, and the rifles chambered in it can handle the pressures. If you're not a handloader it's a no brainier - get a .444, it ain't even close. If you do handload the .45-70 can beat the .444 by a small (but significant) margin, pressures being equal. There are companies that sell hot .45-70 but it's mail order only and very expensive, roll your own and it's no more expensive than anything else. -- Kernel