It depends on the end product and what level of device integration you hope to have (and when you hope to have it).
To start with, it sounds like you’d be best served by designing a standard web application with mobile support… if you are diligent and design EVERYTHING to support mobile browsers then there isn’t any reason that your initial deployment won’t have support for both desktop and mobile platforms on day one!
Another thing to consider, if platform-specific mobile applications are a long term goal, is the architecture of your application. If you plan now to decouple the data and logic from the UI (MVC pattern) you’ll find yourself in a much better position to quickly develop service-consuming mobile applications without having to touch your existing application or server software at all! You can even go so far as to make your web-application service-oriented, which could reduce mobile application development to a simple UI implementation.
The choice of DB really depends on what you need to do, though my personal choice for any “normal” web application would be relational. Here is a link to another question tha thas been answered on the subject of non-relational databases: Why would I want to use a non-relational database?
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solved Mobile or Web application: which way forward [closed]