You don't need an explicit essay on that comparison. Morever it's not vs Copyleft, but why copyleft is generally material to the purpose of Free Software.
In the linked entry Stallman primarily explains what is understood as Free Software. The comparison with Open Source merely serves as a device to illustrate the nuances in a more relatable way.
>Another misunderstanding of “open source” is the idea that it means “not using the GNU GPL.” This tends to accompany another misunderstanding that “free software” means “GPL-covered software.” These are both mistaken, since the GNU GPL qualifies as an open source license and most of the open source licenses qualify as free software licenses. There are many free software licenses aside from the GNU GPL.
I find that distinction very much of interest, and sure enough, RMS also has an explicit text available which deals with that matter more directly [0].
While free software protects the freedoms of the user only, copyleft software protects the freedoms of the developer as well. Both types of software are ethical (as mentioned in [1]), so it comes down to personal choice of the original author, who can use it as a tool to ensure their goals.