In the Greco-Roman period, Coptic was the slang and Greek was the classical language. In the 4th century, Sahidic was the classical dialect, and others were considered to be more local dialects. In the 10th century, the classical dialect was that of earlier centuries.
With the revival attempts of Coptic, all Coptic was considered to be classical - assuming that the presence of Greek loanwords is a sign of a classic language - while the pidgin form that appeared was considered to be colloquial.
It does not compare to the MSA (classical Arabic) v. colloquial Egyptian Arabic as the situation here is diglossia.
