[lmm]

If you find using the same language in the browser and the server to be a compelling advantage, js is your only option. But IME it's not actually that big an advantage; most logic code belongs to one layer or another. For domain objects (i.e. non-logic code) you can use something like protocol buffers and then you can use the same object definition in both languages.

Advantages to me would be much more mature ORM support (e.g. others are talking about how good the migrations are; there are some cool efforts in js e.g. Knex but they're still very primitive in comparison to what Django can now do automatically), better templating (certainly arguable, but I find none of the js approaches encourage proper separation of markup and logic), and a much more comfortable language for writing business logic in. Python has nowhere near as many "gotchas" as js, and is much more readable for a less-technical domain expert.

[emidln]

You can easily do this with Clojure/ClojureScript too. As I understand it, it's also possible with Haxe. Theoretically, you could do the same with Ruby or Scala too (not sure I've heard of other languages with reasonably complete implementations that can handle both compile to native code (or a fast interpreter) and compile to javascript).