Most of the answers (so far, anyway) seem to focus on how easy or hard the choice will be to implement. The whole goal of an application, desktop or web based alike, is to serve the end user. That must be the starting point of the discussion.
Web apps simply cannot provide as rich and interactive experience as a desktop app in most cases. There are just too many limitations due to available widgets, network latency, the fact your app window must (typically) live within another application and so on.
That being said, many times interfaces don't need a large set of widgets, network latency isn't much of an issue, and running from within a browser window is an advantage. gmail and stack overflow are prime examples of web based apps that don't need a particularly rich interface.
So, when trying to decide which technology to use first consider the user. Do they need a high performance, rich UI? Do they need to be able to effortlessly share their data with others across the net? Is this an app they'll use eight hours a day or on just a casual basis? Will they use it mostly to edit local files or to collaborate with people across a network?
For those who say "web apps are always better", let me ask you this question: think about the tools you use every day. Your text editor, programming IDE, your browser, your email client. Which of those are web apps? Probably none, except for maybe your email client. There's a reason why you wouldn't want a web-based IDE for writing software (or writing a book, or doing serious image manipulation).
It all boils down to the user and what their goals are. Sometimes their needs are best met with a web based UI, sometimes with a desktop UI. Sometimes either will do. Of course you also need to weigh options such as distribution and update, cross platform availability, etc. but it all starts with the user and their goals. If you are focused on satisfying your end user the answer will likely become fairly obvious.