IPhone/Andriod/WebOS App or a mobile website?

I'm developing a service that is targeted for mobile devices. I haven't done much mobile dev since J2ME and the Andriod emulator and I haven't done any mobile website development, so I don't know exactly what the pros and cons are.

What are the reasons why I would want platform specific apps over a mobile webapp? What functionality would I want in my webapp that I couldn't get at from a browser?

I like the approach of http://iqmobilesearch.com/ but my project is slightly more complicated.