Try to understand how Qlikview and Qlik Sense differ from each other. Then you need to decide if your customer is better served with Qlikview or Qlik Sense. This blog post might help: QlikView and Qlik Sense. And so might some of the results returned by this search: https://community.qlik.com/search.jspa?q=difference+qlikview+qlik+sense
Desktop applications are slowly moving towards online and cloud presence.(QS)
Saying that QV still giving you more customization out of the box.
The QV script will still work in QS - the UI have to be redesign.
All depends on client and what do they do with the data.
When you start to develop in QS you may found out that you are missing lots of QV features