GRex nailed it. Do what your target market wants. Not a single customer will care that you chose the platform because it was easier for you to develop.
Also be aware, most people on here see web-based apps as the solution for all problems, so expect most answers here to be "web app."
Bill Paetzke's answer includes a reference to the McKenzie's interesting article about why he was done with desktop apps. I read the article, and it always struck me as one very long list of reasons from strictly a developer's point of view of why web apps are better. Not one mention of the customer point of view until the very end, where he switches roles and speaks as a customer:
I love desktop applications. I prefer them to web apps almost any chance I get. You can keep your Google Docs, Excel is superior in almost every way.
He then says how much his users love desktop applications because of functionality web apps do not have (cut and paste that works, double click files to open them, and so on).
If you take away the bias of talking strictly from a developer point of view, the article does make a strong argument that if you have a choice (if the market will accept it), then make a web app. I generally agree with that. So long as we don't redefine problems to fit our solution, we will be ok.
Honestly speaking, I'm happy so many people are focused on web apps. It leaves huge holes in the markets that will not put up with the many limitations of web apps.