I have a love/hate relationship with 44 Mag.
TLDR: 44 Mag in a rifle is complicated. Do more research.
The 44 rifles are all .431 bore diameter unless you get custom because that is SAAMI. Then on top of this, especially in Marlin and NEF/H&R rifles, there is fluctuation in actual bore diameter from .430-.434. That is a wildly large range for modern machinery and manufacturing technology. If you look at the SAAMI specs carefully it will note a +.004 acceptance variance to the .431 bore. So a .435 bore could still be within spec.
This opens up all kinds of problems and extra steps to find which ammo is best for your particular gun. Custom casting and sizing of bullets usually ensues after you have slugged your particular bore.
All this is compounded with the slow twist rates these rifles generally also have. It’s a big headache unless you really love the process. All of this also precludes the use of most factory ammunition which load bullets of .429-.430 diameter and range in weight from 185 gr and generally top out at 300 gr.
Realistically, This leaves little a fair number of options still. For instance, Hornady loads .430 diameter jacketed bullets in 200, 240 and 300 gr XTP, 225 gr Leverevolution FTX, 200 gr Monoflex, These are some of the only factory loaded jacketed bullets that are .430 diameter. There are many lead bullets that are .430 diameter. But really, .431 diameter bullets are the minimum size needed.
So, many folks with 44 Mag rifles are perfectly happy with power and accuracy and everything I have just said will bemuse the heck out of them. I get that but unfortunately my experience has been much worse with 44 Mag rifles.
Be prepared to only hand load and experiment with a number of bullets up to and including custom casting and sizing.
Where does the 45 Colt fit in to all of this. Well, the 45 Colt is in many ways, a hand loader only cartridge as well. When comparing the 44 Mag to the 45 Colt hand loading has to be part of the conversation otherwise the 44 Mag would win every time. The 45 Colt can duplicate 44 Mag pressures and power levels relatively easily with modern components.
If we were only talking about revolvers, I would vote 44 Mag and not think twice. In revolvers, 44 Mag does not demand hand loading to get relative maximal results though you can hot rod 44 Mag if you are so inclined and your revolver is up to it. Hand loading is required for 45 Colt to equal 44 Mag performance. You can get great performance with conventional factory ammunition in 44 Mag and you can get that with 45 Colt with boutique ammunition. 44 Mag revolvers have proper bore diameters and twist rates so not near so many factors have to considered. They are much more plug and play.
In rifles however, all the previously mentioned variables come into play with 44 Mag. 45 Colt rifles however, have no SAAMI standard and as such, are chambered similarly to their handgun brethren. The bore diameter is closer to actual projectile diameter. All of those variables need not be considered when doing load development for a 45 Colt rifle. These rifles also often have slow twist rifling. Most notably, surprise, surprise, in Marlin 1894s and, for me at least, NEF/H&R rifles of which 1-38 is normal.
All these things considered, I think the rifling is the more important variable between these two rifle cartridges. In factory rifles this leaves the Henry Big Boy series and the single shot of which only 44 is chambered in it. These have 1-20 rifling. There are probably a few other factory offerings out there but they are less common.
Every time I bring this topic up, I like to note the 444 Marlin. Ostensibly, a lengthened 44 Mag, the 444 has a SAAMI bore diameter of .430. This is interesting to me since Marlin’s own 44 Mag rifles are .431. More digging into the minutiae of the SAAMI spec for 44 Mag rifle reveals a note about the rifling being 12 grooves and 1-38 twist. This makes me believe that Marlin was the driver for the 44 Mag SAAMI spec since 12 grooves implies Microgroove rifling. This also appears in the 444 Marlin SAAMI drawing.
This brings up a question. Marlin introduced the 444 in 1964. Marlin started chambering 1894s in 44 Mag sometime in the early 60s. The 444 originally had a 1-38 twist. Many folks disparaged this rate of twist and in a fine example of wheels turning slowly, Marlin changed the twist rate of the 444 around the turn of the 20th century. They acknowledged that the higher speed round, the 444 compared to the 44 Mag, needed faster rifling. Faster rounds usually do better than slower rounds in slower rifling yet Marlin changed the 44, deviating from their own SAAMI drawing in the process, but did nothing for the 44 Mag.
Also, SAAMI specs are merely suggestion even if it doesn’t seem like it. They are not a rules exactly. If they were, no one would be able to make rifles in wildcat cartridges. SAAMI is a set of minimum standards that have been vetted. Adherence to these standards will protect a manufacturer from various liabilities of their products.
An example of this is that 7.62 NATO is a cartridge adhering to NATO specs. 308 Winchester adheres to SAAMI specs. These specs are subtly different.