I am running my web service on IIS.
Number of user may be high. May be 100 or 200
There will be frequent updates for long period of time from web service.
Web service does DB call and little calculation
Server specification is not an issue.
Need stability and security in the web service
If you are already using the Microsoft stack (.NET, C#, IIS) on your back end then it would make sense to use SignalR because it will integrate nicely with your existing stack.
100-200 users isn't that many for either NodeJS or SignalR. SignalR uses some of the async features of .NET and so a thread won't be used by an open connection until something is ready to happen (e.g you might be waiting on DB IO). Similarly, all IO in node should be done asynchronously.
If you're using SQL server then you might find using SignalR opens up other possibilities, e.g using Entity Framework to improve productivity and get your product to market faster.
As for stability and security, those are often dependent on the way you design, write and configure your application rather than the technology stack itself.
Edit: A couple of resources for getting started in the respective technologies that I've found helpful in the past:
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