I've owned both and personally found the pro to be FAR FAR FAR superior to the air.
As stated in the other answers the air has the advantage of being a lot lighter and solid state drive is also reliable and faster. However, the air is essentially a netbook competitor. I found mine to be not up to the task for most of the things I wanted to do.
As a designer and web developer I quite often have illustrator, photoshop, dreamweaver and bridge all running at the same time as well as a web browser or two and and FTP program, the macbook air always struggled with even one or two of these applications and the fan would be running virtually constantly if I wanted to do more than surf the web.
With the macbook pro on the other hand, I have Transmit, Illustrator, Photoshop, inDesign, Dreamweaver, Bridge, TextMate, Microsoft Outlook, Telephone, Chrome and Safari all open as I write this post... and the fan hasn't even started.