I believe some research will show the 444 was originally made in 1:38 by Marlin (I could be wrong on this......maybe, Speer refers to 1:38 444's) BUT......anything that can be done with the 1:38 at 1700 fps and slower will only get better as velocity increases. My 44 handled the Hornady 300 XTP just fine, the only problem I had was with the Lee 310 cast bullet, but I'm begining to believe this may have been as much related to an under size boolit, much as anything else. I have read more than a few good reports of the Lee 310 in 1:38 barrels
To Jefferys question.....Get both. No matter how one adds it up a 44 is a 44 and 45-70 is a 45-70. I did a lot of low velocity work with my 45-70, even killed a spike buck with a the Lee 405 HP at a sedate 1275 fps (15 grains of Unique) There is no question in my mind that the 44 is much better suited to reduced velocity loadings than is the 45-70. I'm not saying he 45-70 won't do it, but taking the whole reloading cycle into account, the 44 is a much more efficient low velocity/plinker than is the 45-70, and no matter how you slice it, the 45-70 is a much better heavy bullet rifle.
Two seasaons back I killed 4 deer with my 44, two with a 265 grian cast bullet at 1275, 2 with Speers 270 Gold Dot at 1575. I could not tell one iotas difference between the effectiveness of either load. The deer reacted alike, running some 20-30 yards and dying. One shot each deer, freezer full.
If you are only going to be shooting the factory 300 grain 45-70 loadings, you may as well use the 44, better yet rechamber one to 445, or simply use the 38-55. A 270-300 grain bullet at 1800 fps cares little what bore size it is, and will do essentially the same job. The 38-55 does enjoy a slight trajectory advantage over the 44 or 45-70 but it ain't enough to argue over and it ain't enough to matter in the field or on paper for that matter.
If you are going for the heaviest loads possible, the 45-70 is the way to go, as it doesn't really start waking up till you're throwing 400 grain + bullets. A 400 grain bullet at 1600-1800 fps will stop anything you point it at, but a whitetail just don't need that much killing (to quote DJ from a long time back)
The typical 270-300 grain 38-45 caliber bullet at 1800 fps is going to make something like 1400 foot pounds at 100 yards at at 150 it will still be making something on the order of 1000. The muzzle energy of this beast is nearly 2000 (how does some 1940 ft. lbs grab ya) That's a lot of swat on a 120 pound deer at 40 yards....you will have meat in your freezer.
Does that help with the perspective?