For QlikView server the QlikView WebServer use the webserver components which are already a part of a Windows Server which also Microsoft Internet Information Server (IIS) use if it is installed. So performance and quality-wise you should be well off using the "QlikView WebServer" as long as you don't need the extra features that IIS has to offer. Some larger customers are already using IIS and are more comfortable selecting that option. Another reason for going with IIS is that you can more easily do server-side web development.
