Any particular reason this is better than Mou? The only advantage I can see from the website is the tabbed file viewer, but I guess I've missed something. (10.8+ seems a disadvantage over Mou, it has versions for 10.6 & 10.7)

Mou is not a very serious editor. It's just convenient because of the live reload and some Markdown highlighting/styling. You quickly get insane working with the lacking text manipulation features and the depressingly broken undo/redo.

The reason I use Mou is for the LaTeX integration. Haven't found another editor that does that.

It's just using MathJax.js

I have a friend who uses Mou daily, and he complains about Mou leaking memory. Also, Mou is for 10.7 and up only.