Lots of factory ammo available for both. I'd rather shoot the .45 colt. The .44Magnum has a bit of punch. Magnum ammo is easier to find, and a little cheaper. I love the .44 magnum. Great all around cartridge for self defense, hunting, target practice, etc.
Recoil isn't unmanageable with either by any stretch. Bountyhunter really covered the essentials so there isn't much left to add. One of the benefits of the .44 Magnum though is you can fire .44 Special ammo out of it as well.
Standard factory ammo for a 45 Colt is a 255 gr lead bullet driven to about 860 fps. Because there are so many light frame 45 Colt revolvers in service, the ammo is not nearly as powerful as it could be with a strong gun. With a Ruger Redhawk, Blackhawk, or the heavy frame Ruger Vaquero, you can easily load 45 Colt ammo to 44 Mag power level and beyond. Buffalo Bore makes some heavy duty 45 Colt loads but I don't have the data handy.
Recoil is always a function of muzzle energy .... just like the old 5th grade science class taught "for every action there is an equal and opposite reaction". Assuming the weight of a 44 Mag revolver and a 45 Colt revolver were the same, the 45 Colt with standard factory ammo will have very modest recoil. That same gun with "Ruger Only" 45 Colt loads will recoil much the same as a 44 Mag ... maybe more.
Here's some examples:
45 Colt factory load = 255gr LRNFT @ 860 fps = 419 ft lbs energy, 31.3 lb-f/s momentum
44 Mag Factory Load = 240 gr JHP @ 1350 fps = 971 ft lbs energy, 46.3 lb-f/s momentum
45 Colt Ruger Only hand load = 250 gr SWC @ 1350 fps = 1012 ft lbs energy, 48.2 lb-f/s momentum
As you can see ... a 44 Mag or a 45 Ruger only load is going to make a big dent in a soda can ... ouch!
The standard 45 ACP will have a 230 gr FMJ bullet driven at 850 fps. Considering the 45 ACP is a lighter bullet (230gr vs 255 gr) driven at about the same velocity, that would make the 45 ACP slightly less powerful and slightly less recoil.
Ruger does make a 45 ACP revolver but it is in a single action Blackhawk or Vaquero and is available only as a convertible (45 ACP / 45 Colt) with a dedicated cylinder for each cartridge. Here's my heavy frame Vaquero Convertible:
If you 'load up' acp's, keep an eye on the extractor groove. That's the thinest area and I have had them split on me. If your feeling brave, cut down some 30-06 brass. Turn that 1911 in to a magnum! Not for cast frames!
deputy125, Ruger makes an assumption related to the 45 Colt and that is .... most shooters will use .451" jacketed bullets so they set the throat diameter to .451". Fact is, most factory ammo is loaded with .454" diameter lead bullets. Now we have an issue ... the throat will swage down the bullet to less than optimum bore diameter. This results in a loss of accuracy and an increase in fouling. To top it off, Ruger often ships their 45 cal revolvers with tight throats ... well under .451". This makes the gun even less accurate and makes it foul more no matter what bullets you use.
The fix is simple ... chamfer the cylinder throats to .4525" to optimize the revolver for lead bullets. If you plan to shoot mostly jacketed bullets, .4515" is the optimum throat diameter. I will give Ruger credit for their bores ... I have seen several rough ones but I have never seen one that slugged more than .452" or less than .451" with .4515" being the norm.