Portable headphones induce NO impedance mismatches as most are above 32Ω, some much higher. The Continental does better with higher Ω headphones, hence the reason I said it shines with full size and portable headphones. If impedance goes too low for the Continental, it starts to have a different sound signature and loses some of its power. Sweet spot is 64Ω and higher, though 32Ω is great, too.
Maybe it would be best to say it like this: both amps are great, but if you are using earphones, the National is better. If you are using headphones/portables, it is up to you. The National has a more squeaky clean signature, the Continental more of a tubey/warm signature, which is best used with phones of more than 64Ω, though down to 32Ω is good too. At that point, it is up to you which you prefer: one does NOT drive necessarily headphones better than another, but the Continental has its unique sound signature that the National mainly steers clear of.
Preferences, that's all, as it pertains to headphones/portable headphones.
I see what you're saying now. So most headphones (portable and full-size) have an impedance of greater than 32 ohms, which fulfills the 1:8 impedance rule of the Continental. Since they fulfill the same rule for the National, the choice is mostly based on sound signature. Thank you for patiently explaining that.