A .45-70 is more versatile than a .44 magnum in that it can be loaded down to .45 Colt levels and up to nearly the level of .458 Winchester factory ammo. If the .44 is enough you don't need more, and the .44 has a greater magazine capacity due to the shorter cartridge, and is a trimmer handier rifle. The BLR is mechanically complex, to the point that Browning warns that disassembly should not be attempted, which is something of a red flag for me. They do carry nearly as nicely as a traditional lever gun, and a .308 is flatter shooting than either the .44 or the .45-70, and can be loaded with a broader range of bullet weights and types, which speaks well to versatility. If all 3 rifles are in similar condition, it pretty much comes down to personal preference. I'm curious though, why did you initially buy the BLR chambered for a very popular rifle cartridge, but now think a carbine loaded with pistol ammunition suits you better? Are you dissatisfied with the BLR or the .308, or do you have other .30 caliber rifles, and now find the Marlins more appealing?