That's a great question and something I plan on writing a full blog post about. But, from a high level, here are the projet goals we've been working with:

1. An administration interface must be good looking and easy to use. The interface is not for developers who understand the underlying data model, rather for operations staff who need to do their job and service our customers.

2. Administration of a production application is way more complex than a simple CRUD of database tables. Although editing data is necessary, most of the time operations staff are finding a resource and performing an action on it or looking up historical data.

3. An administration framework must be highly customizable. Active Admin makes it easy to add controller actions, sidebar elements, action buttons, dashboard widgets, custom forms, show screens and hook in to all resources via plugins.

Right now it looks* like rails_admin is a bit more feature complete as far as CRUD administration goes (+ history of edits). While ActiveRails has feature like comments which may be more relevant to less technical users.

* Disclaimer: I'm more familiar (and probably biased) with rails_admin

I would love to know what the differences are too. Although, to be honest, I have stopped using RailsAdmin mainly because of deprecation issues.

Also, it wasn't as nice as I would have wanted.

But this one looks really nice.