If we design a web app (simpler and standard technology, many mobile websites are being developed in HTML5 / CSS3,) we will have to take into account that the designs have to be Responsive Web Design. We can use any tool to make our prototypes but using java script frameworks will help us a lot and facilitate the subsequent implementation. That's why here if the UX designer knows these languages well, it will give a lot of added value to the project.
On the other hand, deciding on one approach or another will depend on the business objectives and understand the characteristics that differentiate web applications or android application, native applications, hybrid and responsive designs. In any case, you do not always have to choose between one and the other. Web apps and native applications are not competitors and can complement each other. My opinion, as of today, is to first have a quality website that offers a good user experience (there is still a long way to go) and then focus on offering added functionality through a native application.
