New guy here; my first post, so take it for what it's worth. IMO, while "energy" figures aren't the only relevant factor, if you're comparing same-diameter weapons as we are here, energy numbers do shed light on some things.
If stuck with those two caliber choices, I couldn't say. The .45-70 is more common, and therefore more readily found in stores. The .450 is less plentiful, but more capable.
If I may suggest another route... a lever gun in .454 Casull. With full-power loads, it's got more power than the .45-70. (Don't know the .450's actual energy numbers; it's not listed even in the 2004 shooters' bible). But with top-end loads, the .454 breaks 2,000 ft/lbs in a revolver, with the revolver's cylinder-gap loss and one-third-the-length barrel (7 1/2" vs. 20 inches); so it should easily break 2600-2800 in a closed-breech rifle or carbine.
Other things the .454 has going for it are "same-caliber" rifle/pistol combo capability, and higher capacity in the rifle.
But the most impressive thing, imo, is versatility. Even sticking with factory ammo, you can get power levels of around 2,600-2,800 ft/lbs with top end loads, 1200-1600 ft/lbs with "medium" .454 loads, 600-900 ft/lbs with upper-end .45LC loads, and even down to 300-500 ft/lbs using normal or "cowboy" .45LC loads. So it's good for game from elk to rabbit, just by virtue of changing the round in the chamber; no conversion kits, no adaptors, etc.
Don't know if CCI still makes their shotshells in .45LC, but that would nearly make it a .410 shotgun as well.
If you reload (which you would almost certainly have to with the .450 anyway), you can tailor loads anywhere in the spectrum from 200 to nearly 3,000 ft/lbs. All in the same gun; that's versatility.
Plus, (in the Rossi, anyway) it's in the more "traditional" 1892 design, rather than the newer styles.