It's about what you want. With the air, you get the thinner door-stop shape. With the pro, you get something thicker but still one of the thinnest laptops around.
The pro gives you a much more powerful processor, the ability to upgrade to an industry-standard SSD which will run rings around the slow proprietary one in the air and let you easily and cheaply upgrade it later. And with the pro you can upgrade to 16 gig of memory later whenever you feel you need it and currently for about $80. The pro also gives you a second drive bay that comes with a DVD burner but can be replaced with a hard drive. An MBP with a 256 gig SSD *and* 1 TB hard drive is pretty sweet and costs less than upgrading the air SSD from apple.
So the question is if you want a skinny door-stop with little power and no future or a still-thin powerful and upgradable machine.