Howdy
You are correct, both rounds are very similar in performance. I shoot 44-40 in my rifles in CAS and 45 Colt in my pistols. You will hear folks advising against doing this, as a 45 accidentally finding its way into a 44-40 rifle will jam up the gun and require it to be taken apart to get the round out. All I can say is I have a pretty good system to prevent that from happening. I shoot 44-40 in my rifles because when I started the game I had a couple of 100+ year old rifles and in those days rifles were never chambered for 45 Colt. But I had been shooting 45 Colt in revolvers for years.
Anyhoo, when I load the two cartridges I use the exact same powder charge in both of them. Whether Smokeless or Black Powder, I use the exact same charge. This means both cartridges will have the same amount of chemical energy locked up inside. Since my 44-40 rounds have a 200 grain bullet and my 45 Colt rounds have a 250 grain bullet, if I was to shoot them in identical length barrels, the 44s would shoot a little bit faster and flatter than the 45s, but they would both have the same potential energy.
By the way, just for the fun of it last year I finally got around to shooting a cylinderfull of 44-40s out of one of my 45 Colts. We always talk about how the bullet would rattle down the barrel and shoot accurately, but all 5 bullets hit the target just fine.