First thing you need to check is what twist rate you have do in each barrel / caliber. Next, do you want better accuracy or more knock down power and last do you want a bigger range of yardages where you will be close without a lot of hold over / under etc.
An 1873 Trap Door Springfield in .45-70 can Lob a 305 grain projectile into a small target area at 1,000 yards, but at a few yard before or after may be way off. The flatter shooting .270 can do better than the .30 caliber. For knock down power, you need to match bullet weight and design to the barrel twist an length. Depending on what you have you might get the same muzzle energy when considering velocity times mass or each.