The default renderer is too "loose" on OS X 10.8 whereas the CoreText renderer looks the same across all platforms. Furthermore, the CoreText renderer is many times faster. A downside to the CoreText renderer is that it does not support the text input protocols properly, so e.g. three-finger tapping to show dictionary results does not work. It also does not show marked text as it is being drawn if the "inline marked text" option is disabled.