Affects the ffmpeg post processors, if ‘--prefer-ffmpeg’ is given and both avconv and ffmpeg are installed, it will use ffmpeg. Otherwise it will follow the old behaviour.
On Windows, double quotes are required, because single quotes get served to youtube-dl. (Yes, cmd.exe is crazy like that).
On other system, both double and single quotes are fine, unless the string contains a dollar sign (then you need single quotes).
Since virtually no URLs contain dollar signs, double quotes should do.