different between c# and java to develop android

The important question to ask here is "Do you need the same application to be ported on multiple platforms? (Android/WP/iOS)"

If the answer to this question is yes then C# then Xamarin is a very good choice. Keep in mind that not all the code will work on all the platform, You will have to rewrite some parts of the application for each platform. There will be some code that will be reusable across platforms if you are using Xamarin.

If you can live with a non-native application then perhaps HTML5/JavaScript is the best choice. Nowadays this is becoming the default choice for most of the business apps which need the support for multiple platform and can compromise a bit with performance.

Now if you are only interested in developing android Apps then perhaps you should use Java. Java is not that different than C#. It is an elder brother who lacks a lot of features(elder brother got complacent perhaps). But using Java is the best choice in this case.

Ultimately the choice of language should be based on application requirements and not on personal preference. I personally am in love with C# but have written Android apps in Java and HTML5 too because my applications had some requirements for that.

P.S. based on your application requirements, I would suggest go with html5/javascript. Since you need to communicate to the server, I would say using Backbone marionette framework is the best option for your application. I would have chosen it.