from your experience, why do you think it's better to choose the web as a platform for such application?
It completely depends on usage and deployment scenarios.
If you want a single point of deploy and don't mind having less control over the UI, use a web application.
If you want tight control over UI and would rather deploy to multiple machines (for each HR personnel), use a desktop app.
From your question, you have already decided that a web app is the way to do. Why bother asking then?
I can't think of any aspect intrinsic to Human Resources applications that would lead one to prefer Web or Desktop. It's all down to the exact requirements, both functional and non-functional.
For what it's worth, I observe that the trend generally seems to be to use Rich Internet Applications because a): you provide pretty excellent UIs in Browsers these days and b): the deployment pattern is so much simpler. But there are still specialist applications that are quite hard to develop in the browser (for example CAD-style drawing apps - yes they can be done in a browser but it is quite tricky to do them).
Also there are interesting hybrid environments such as those offered by Lotus Expeditor where there can be a small desktop footprint providing a local web server and database allowing off-line working while still running a RIA. [Disclaimer: I'm IBM, I work with this product.]
Well, the first difference is that you only need a browser to work with a web application so basically your user can work from everywhere. I think you'll save money in the IT department because you do not have to update every single machine but just your web application. But this is just a general thought. When you choose a specific tecnology you can have more advantages or disadvanteges to take care of.
A desktop application in my opinion is useful if your software, to work, needs to take advantage of the client resources.