Howdy
I'm glad CraigC corrected the misinformation about the size of the 1860 Army.
The grip is indeed only about 1/4" longer than the 1851/1861/SAA grip. I too ALWAYS grip a SAA type revolver with my pinky under the grip. That is how it was designed, no point trying to cram the entire hand onto the short grip. With the 1/4" longer 1860 grip, it is too big for me to get my pinky under, and a little bit too small for me to cram my entire hand on. With my pinky under the 1860 grip, I have to regrip in order to reach the hammer spur to cock the hammer. With my entire hand on the grip I can reach the hammer spur, but I have to regrip again to shift my hand down just a smidge so that I have good trigger control. If the grip was another 1/4" longer, it would be good for me, but it ain't. I strongly suggest trying both grips, and going through the entire exercise of cocking the hammer, then seeing if the hand needs to shift in order to shoot the gun comfortably. Don't drop the hammer, just ease it down. See which one fits best, both with the pinky under the grip and with the entire hand crammed onto the grip.
The frames of the 1851 Navy, 1860 Army, and 1861 Navy are all exactly the same size. What Colt did with the 1860 Army was take the Navy 36 caliber cylinder, and add metal to the front end, in order to fit six 44 caliber chambers in. but the rear of the cylinder where the nipples are remained the same diameter. The frame then had a relief cut to accept the larger diameter of the front of the cylinder. But other than that relief cut, the frames are exactly the same size.
I'm sorry to say this but for a first-timer I'd get the Remington. That or get very good at knocking out the wedge that retains the barrel and putting it back together. You'll want to wipe it down every other cylinder full to keep it clean and operating. Why the Remington? Pull down the loading lever, pull out the pin, and the cylinder falls right out. Nothing kills the enthusiasm like a gummed up revolver or one that's stuck because a piece of cap fell down into the lockwork. Black powder revolvers are like playing golf. You'll have very good days and others, not so good.
Sorry, but I disagree. Of all the Black Powder revolvers, the 1858 Remington is the worst for binding up. That is because 1. there is no cylinder bushing to keep fouling blasted off the cylinder pin, and 2. the pin is very narrow and binds up quickly. While Colts also lack a cylinder bushiing, the cylinder arbor is much larger in diameter, and that spreads out the fouling more. In addition, the Colt design incorporates a helical groove running around the arbor which creates clearance for fouling to be deposited in. A properly lubed Colt open top type C&B will run longer without binding than a Remington, at least that is my experience. I can shoot an entire CAS match with my 1860s without wiping down anything, or driving out the wedge. With my Remmies, I have to stop and wipe off the cylinders and the cylinder pins every other cylinder full or they will bind up. Of course, the mechanics of the Remmie does make this easier to do than with a Colt type.