I might disagree with my friend above. Starting with a caplock rifle is like learning to drive with automatic transmission. A flintlock rifle is like learning to drive with manual transmission.
If you can drive "stick", you can drive anything else. Yes, a flintlock is more involved, and requires more patience and understanding. But again, once mastered, you can handle shooting any caplock, which you would find likely, a waste of money.
A flintlock does not require the shooter to be concerned about the availability or selling price of percussion caps. There's no HazMat charge on a purchase of a dozen flints.
There's nothing wrong with either choice. But flint shooters are held in high regard for the apparent handicap, unknown to them, they seem to face.
As noted here, your best bet for an accurate beginner's rifle is the Lyman Great Plains rifle in .50 & .54 caliber. Lyman offers the widest choice of opportunities in their line; left-handed, right-handed, flint, percussion, & kits to build.
Being in the muzzleloading firearms business since 1972, the Lyman G.P. rifle has been my best seller, new & used, and will cloverleaf groups at 50 & 100 yards.
"Flinters" are folks who LIKE to tinker, & piddle with their rifle.
Flintlock hunters and competition shooters, prior to shooting, have to check at least a half-dozen "points" before pulling the trigger; 1) flint must be sharp, 2) flint must strike frizzen square across frizzen face, 3) left side of flint must not scratch the barrel, 4) flint must strike the frizzen between 3/4 to 2/3 the height of the frizzen, 5) vent must be clear for spark to ignite barrel load, 6, flint must be tight in hammer jaws.
These "points" before each shot can make a caplock shooter mutter under his breath, "dang glad I don't own a flinter".
It mainly depends on your patience level; like learning to drive that Chevy with the "stick-shift".
If you master the flintlock, shooting a caplock will be a breeze, not that you would want to anyway. Once a "flinter", always a "flinter".
'nuff said.