Erato has a more flexible window layout than Mou, and provides some nice editor shortcuts that programmers are used to (e.g. better identation features and block autocomplete).
It would be nice if that was mentioned somewhere. Looking at the website now I just see a Mou clone with less choice and no customizability.
Erato doesn't change the text size in the editor window, just in the live preview window. Mou is clever in this regard but its cleverness that doesn't improve the experience IMHO.
This is optional. Mou has several editor themes and at least half of them don't do this—only the ones that end in "+" do.