I'd say more info is needed. Do you want to be able to hunt with the rifle, or are you interested in Cowboy shooting, or both? The .45 Colt does not have the energy of the .44 magnum, after all, if it did, Mr. Keith would not have had to create the .44 mag eh? Many states require you use a "magnum" cartridge for hunting. If you do choose the .44 mag remember that you can also use .44 Special and .44 Russian ammo in the revolvers and rifle as well. AND..., if you're setting up to handload the .44 mag already, you will be able to load .44 special too, (perhaps also the Russian) and at a very reasonable price per round.
The .44 Russian was the first of the three, and was a well known, accurate, target and defense round. The .44 special came about as an alternative, as I believe the .44 Russian was patented, and the rights to making a handgun for that round were owned by a single company. The .44 Magnum was developed from the .44 Special as a hunting cartridge for handguns.
There is also the .44 Desert Eagle and the Ruger .44 magnum semi-auto rifle, lots of .44 lever action rifles, as well as the single shot T/C Encore, which makes the .44 magnum a really well rounded choice in firearm availability.
After all that, I have a pair of Ruger Vaqueros in .45 Colt, as I like the nostalgia of the old round. That's just me. I can't hunt with my .45 Colt rifle, unless I handload a really HOT cartridge. BUT as I live in the PRM, the places that I hunt don't let you use anything but a shotgun or BP rifle anyway.
I hope this helps
LD