One thing we haven't brought up here is the fact that it's much easier to lump sidelocks together than it is boxlocks. Relatively few truly different sidelock mechanisms on guns made over the last century; a whole bunch of different boxlocks. (Each of the "American classics" was a different design.)
The point that one may prefer sidelock triggers to boxlocks because a higher percentage of sidelocks are higher grade guns is well taken. And there are certainly inexpensive boxlocks with good triggers. However . . . if boxlocks are really a better design, why do some makers--Francotte perhaps most famously, but also V. Bernardelli--make their boxlocks to look like sidelocks?? That would seem to indicate that sidelocks certainly have an advantage when it comes to "pretty".
And you have to love Cogswell & Harrison's euphemism for false sideplates: "ornamental strengthening plates".
