Should You Build a Mobile Website or App?

I've been working with a team to build a mobile app for Maritz in the MPrize competition. One of the first debates was whether to build a mobile app, or a mobile website.

I argued that a mobile website would be easier to implement and be available over multiple platforms all at once. Mobile apps are great, but they take a lot more effort to put together. Then you have to get people to install them, and keep them updated. The team decided to go with the mobile website instead of an app.

Today Mashable posted a report of how the top Twitter client Tweetdeck is building a mobile web interface, which makes it sound like I may have been right ;)

The article goes over in more detail the many reasons why they chose this path.

What do you think is the best approach?