the MacBook Air is a great laptop, but it's not as versatile as the MacBook. Mainly, it is designed to be an ultraportable Mac super-netbook, which is to say, it includes features, like a core 2 duo, larger screen, and larger keyboard that normal cheapo netbooks lack. It's drawbacks are the lack of an internal superdrive, fewer USBs (for those of us who use USBs), and it's slower Core 2 Duo (the standard MacBook quite a bit faster than the standard Air). Essentially, it is a 'carry everywhere' machine, and as such, has compromised on performance and versatility. I would advise using it as a portable machine, not my primary computer, but then, some people are happy with netbooks as their primary computers, so don't take my word as final!
The regular MacBook is ultimately a much better all-around machine. It has a DVD drive (of course) as well as a much faster CPU, and a decent feature set. It lacks some of the finishing touches, like the backlit keyboard and aluminum body, of the Macbook Air. It's a plain and simple computer with plenty of power and the basic features you'll need.
Personally, I would suggest neither of these laptops, but rather a MacBook Pro 13". It's got the nice touches included in the Air (aluminum finish, backlit keys), plus the features of the white MacBook (faster CPU, built in DVD), and even a few unique features of it's own, like the firewire port and larger memory. It looks more or less the same as the Air, only weighs 1.5LBS more, and actually costs less. So, it might be an acceptable compromise.
Here's a link to a comparison on Apple's website: http://www.apple.com/getamac/whichmac/whichmacbook.html
hope this helps!