My 637, whose sight insert was recently featured in another post here, was previously painted. I also had another painted snub to A/B against the new insert. (BTW, the same bottle of Testors was used for all of these.)
I see no advantage to the insert over the paint in appearance. The paint may be ever-so-slightly better because the insert is not as shiny after filing to shape.
However, after about 3 years of pocket-holstering, the previous paint on the 637 was pretty dark. I’ll have to see how the insert holds up by comparison. I suspect it will be longer lasting. But it was also more work, and not “undoable”.