Get the MBP unless you want to be buying a new Air in a year or two when you get totally frustrated running Lion and any apps on 4 GB of RAM.
The Air is an attempt on Apple's part to force people to replace their laptops more frequently. It's a sealed system--there is, quite literally, nothing that is user upgradeable, compared to the MBP, which not only starts out as a substantially more capable laptop but can be upgraded by the owner down the road.
The Air demands a huge price premium in exchange for giving up a lot of capability, in exchange for which it is a bit lighter. If you're willing to do it, fine, but I think it's absurd.