What a dilemma!
If you have some 44 Mags already, you might want to stay with the same. If you have 45 Colts, you might want to keep to that caliber. But then, if you have one of these, maybe you should get the other so check out that caliber.
Unless you reload, the 44 mag will be cheaper to shoot with a more plentiful ammo supply. The 454 is more powerful in factory loadings with a larger diameter, thus being arguably more "deadly" on game. Either will kill anything on this continent if we do our part, but I read once that Ross Seyfried said that using the 44 Mag on some African game felt like sending a boy to do a man's job, but the hot-loaded 45 Colt in a custom 5 shot was (joking) like a short barrelled 458.
You can shoot 44 Specials in the 44 Mag, and 45 Colt in the 454 if you do not mind scrubbing the "ring" out after shooting the shorter ammo. But then, neither of those sub-rounds are terrible common at WallyWorld, either.
If you handload, you can go from cream-puff loads to ???? but the 454 will still have more potential at the top end.
Winchester, Marlin, and Ruger make 44 Mag Rifles, if you have a yen to carry both wheelgun and rifle in the same caliber. I would think someday one of them will catch on and come out with a 454 in a levergun.
Maybe the best thing to do is help the economy recover and buy one of each. But then the question is still, which one first. But like so many of our choices, it would be hard to go wrong on this one.