Wanted to have some views on which one to build first if a web app has both?

Feature wise it would be like a simple list type of app (you could think of it as something that looks like HN) which is neutral to the platform, i.e it doesnt really matter in terms of user experience whether usage on web or mobile.

I have read that some people advocate using web application versus mobile development because its much easier to identify and solve problems and bugs, easier to write codes and build up a more scalable system. Also, mobile has more updates and crashes. Ruby on Rails for example can allow for both web/mobile especially with CSS Bootstrap.

On the other hand, some say mobile is easier because you have things like html5 which allows for cross platform codes as well as being easier to build. One can just use JavaScript, CSS and Html to bash something from say PhoneGap while on SQLite.

I have not arrived at anything conclusive and Im quite new to programming. I did look up how guys on both platforms built (such as Instagram, not because they are famous but they reveal their stack publicly :) ) but again no great insights either.

Any thoughts on the pros and cons, suggestions and questions I need to ask myself? Help is much appreciated! Thanks!!