For the first-time user, I'd suggest a stainless steel Remington in .44 caliber.
Here are my reasons:
1. Stainless steel will rust over time. I had it happen to a Ruger Security Six .357 Magnum years ago. But it takes quite a bit to rust stainless steel and it's more forgiving to those who put off cleaning their revolver. Old-timers like me (been shooting cap and balls since about 1970) tend to clean right away but newcomers to the sport often neglect this requirement.
2. The Remington has better sights than the Colt. This means more hits on target.
3. The sights are more easily altered to hit to point of aim. If the Remington hits low, file down the front sight a lick at a time. If it shoots to one side or another, drift the front sight in the opposite direction of where you want the ball to strike. Altering the Colt's sights (a notch in the hammer and a brass bead near the muzzle) takes more modification, best done by a gunsmith.
4. Strength? Either the Colt or Remington are plenty strong for black powder and black powder substitute pressures. I consider this argument specious.
5. I suggest .44 caliber because cap and ball sixguns work best with oversize balls of .454 and .457 inch, instead of the oft-recommended .451 inch. If .380 inch lead balls were as readily available, I'd suggest the .36 caliber since it uses less lead. Most sources recommend the .375 inch ball but I find it too small for cap and ball chambers and less accurate.
6. The .44 Remington may be had with modern, adjustable sights. Not so the Colt. While this is not authentic, it can help the plinker and informal target shooter to make more hits, and this breeds confidence. Besides, it's fun hitting what you aim at, eh?
Weaknesses of the Remington design:
1. If you put in too much powder, and can't seat the ball below the chamber mouth, it's a task to rectify the situation. With the Colt, you simply pop off the barrel assembly and fire the ball directly out of the cylinder. I've done it myself, without incident.
2. The Remington doesn't have as much room around the rammer as the 1860 or 1861 Colt. However, the 1851 and earlier Colt models are just as cramped as the Remington. This doesn't make a difference when using balls, but trying to seat a conical bullet can require juggling.
3. The cylinder pin on the Remington is smaller in diameter than the Colt's. It's also smooth, whereas the Colt has rings milled into it, for fouling to migrate into. The Remington's smooth cylinder pin leaves nowhere for fouling to go, consequently it builds up and causes the Remington to drag and foul faster than the Colt.
4. I think everyone will agree that the Remington is nowhere nearly as well balanced as the Colt design. The Colts point and handle very well. Through the years, the Colt 1851 Navy has been hailed as the most perfectly balanced handgun ever invented. The same could be said for its improvement, the Model 1861.
5. The Remington has fewer parts but its mainspring must be compressed for reassembly. I typically gently grab the mainspring with Vice Grips and move it into the frame. The Colt is easily reassembled without any spring compression. However, I must admit that the Colt has more screws likely to be lost.
6. The rammer on the Remington doesn't go as far down into the chamber as the Colt. This means using fillers or felt wads if you use light loads, so the black powder (or substitute) may be properly compressed.
There should NEVER be a space between the ball and powder. This is a dangerous situation that can cause an enormous pressure jump and blow a gun. The Colt reaches down a little farther into the chambers but you often still have to use a filler with the Colt too. I use corn meal or greased felt wads for fillers, between the ball and powder.
I own Colt and Remington designs, in .36 and .44 caliber. I like them both. For the beginner, I suggest the .44 Remington in stainless steel with .454 or .457 inch balls.
If he decides he doesn't like cap and balls, he can sell that revolver without much problem. If he does like shooting cap and balls, he can go on to other models. Yet, that stainless steel Remington .44 is still mighty useful.
If someone made a Remington stainless steel in .36 caliber, and .380 inch balls were readily available off the shelf, I'd suggest that, instead. The .36 uses less powder and lead yet it's just as accurate as the .44 if properly loaded.
Buy a stainless steel Remington. If you like the hobby, get a Colt 1861 Navy in .36 caliber. The 1861 is not as famous as its predecessor the 1851 but it's a better design and it's still authentic. If you wish to stay with .44 caliber, get the 1860 Colt.
Both are good designs in their own right.