It really comes down to this...
The Pro will be slightly faster, has a built-in DVD, and is way more upgradeable. Do you want this over the portability of the Air? Can you afford the extra cost?
There is no one over the other for everone. The decision is partially subjective, and partially objective, depending on both the users needs, and wants.
Just got an Air and I can't believe how light it is! Of course you can see the ads and pick it up in a store... what I mean is that the difference it makes in my computer bag is just unbelievable. I am SO happy that I'm not lugging around all that weight anymore. I really love this machine!
I bought my wife a 13" MacBook Pro and my mother has the 13" white plastic MacBook. They're both fabulous, but I'm just extremely happy with this lightweight laptop. I have a ton of software -- Windows 7, MS Office, etc. Performance is great. It boots in the blink of an eye.
Sorry, I know, I probably sound like too much of a fan. I've wanted one of these for a long time, and I'm surprised, really, that it exceeds my expectations.