Flint is #1 for me, but on especially wet/rainy/melting snow dripping from the trees days I get lazy and take a cap-gun. I can keep a flinter running in the wet, but, "LAZY" being the key word here. And I'm a big fan of the Civil War rifles, and try to take them out for a hunt once in a while, so they don't feel neglected.
What I like about a flintlock, is that you can see anything that might cause a miss-fire. It's a visual thing. Unless you pull the nipple, you can't see inside the nipple, or flash channel, bolster, drum or whatever. With flint I can feel if the flint is loose, see that it's set right, see into the touch hole, stick a pick in it and feel if the powder is dry and crunchy, or wet and mushy, see if the pan charge is good, or wet, or absent, see if the frizzen is wet, etc. etc. etc. That is why, for me, the flinter is more reliable. Having said that, it's been 30 years since I've had a miss-fire with a cap gun, but that's my story and I'm sticking to it. "Peace of mind" I guess.