The primary visible difference is the way hinting works in both formats. TrueType-flavored fonts (.ttf) must be explicitly programmed; how good this looks depen

The primary visible difference is the way hinting works in both formats. TrueType-flavored fonts (.ttf) must be explicitly programmed; how good this looks depends on in which decade the font was hinted and how much money, time and skill went into it. Postscript-flavored fonts (.otf) can be outfitted with some hinting metadata and have an automatic hinting tool run on them, applying the hints is then completely up to the renderer. This has the disadvantage that the designer has less control over the end-result and the advantage that the renderer can interpret hints differently if some text display technology/stack changes. I personally find Adobe's .otf renderer in FreeType the best compromise between aesthetically pleasing and legible and can only recommend using exclusively .otf fonts on FreeType platforms.