We use both Rails and CodeIgniter. Rails has a much more useful core and its libraries are more robust and useful. That means you will be able to develop more quickly. However, you will have to spend some time learning how it works if you've no experience, and you will want to build your app the Rails way which might mean some redesign (hopefully not too much.)

Here's what you should look into: auth: devise gem, crud: Rails defaults + mongoid gem, rest: by default in Rails.