Personally, I believe if you can render JSON in a structure that the Ember app expects then it really doesn’t matter. The primary considerations are performance, hosting and deployment, manageability, and perhaps most importantly “developer happiness”.
It is my impression that the core people behind Ember come from a Rails background so there is likely implicit bias in the conventions and they are informed heavily by Ruby and Rails patterns.
That being said, there really is a clean separation between client and server. In a lot of ways a more pure separation because the server only has to render data structures (JSON) and not (too much) markup HTML.
It is probably worthwhile, to explore the server side technologies and languages independently to get a a sense of the practical tradeoffs. A lot of intangibles. A lot of this is personal feel. What is it like to live within an environment? What is the documentation like? What is the community like when you need help? Do you have co-workers or friend who you bug when needed? What are the build and deployment tools like? Over the years, I have programmed in Perl, PHP, Ruby and some Python.
There are a lot of things to be said about Python with its whitespace syntax. The code is generally easy to read. It is kind of hard to write insane obfuscated code that used to be the hallmark of Perl. But these days I tend to appreciate Ruby. There is an amazing number of “meta programming” type things you can do. Which for me seem useful.
If you just want to work locally and want to quickly prototype some backend ideas you might consider checking out lighter weight tools
The thing I hear many people mention is that they tend to come back to Rails because it has a lot of conveniences baked in.
My two cents.