It would seem for whatever reason I managed to ignore/miss this thread for the longest time. At least until it is actually too late because Doug.38PR already made a decision and bought his rifle.
However ... It's Monday and I'm bored and my Dog wandered off down stairs and I'm alone and ................
There are several considerations for this question. Mostly from purely mechanical/practical stand points. First up is the .45 which is actually more versatile than the 44-40. With the 45 you have a choice of cartridges, 45 Colt, 45 Schofield and C45S (needs a carrier). The main drawback is Blow-By. With 45 cartridges, you get blow-back. The case will not expand to seal out modern chambers. The chambers are simply too generous. Blow-By results in crud building up on the Carrier Block and in the Mortice. A goodly amount of cleaning is required and some rifles will need a squirt of something in the middle of a CAS match to keep running. From a personal prejudice vein, NO lever rifles were ever charred in 45 Colt in the time period we portray.
44-40 or 38-40. The only versatility with the 44-40 is bullet weight and fillers for reduced charges. With a 44-40 you have a 44-40. No other cartridges will work. 44-40 is normally Clean, Free Running and Sanitary. The case seals the chamber so there is seldom any Blow-By at all. Normally the Carrier Block and Mortice will be as clean at the end of a CAS match as it was at the start. A Patch or two thru the bore and your done. From the personal prejudice vein, the 44-40 was the introductory cartridge for the 1873 Rifle. Historically correct.
If your desire is something resembling history, your only choices are 44-40, 38-40 and 32-20. If on the other hand, your crazy like I am and just can't resist messing with your rifles, go with the 45 and learn to put up with the Blow-By.
Burma Shave
Coffinmaker