I thought I might respond to your question, as I recently struggled through this very difficult decision. This was my first Mac purchase ( I had wanted one since I was a kid, just never had the $).
My choice, due to budget constraints, was between either the base 13" MBP or the base 13" MBA. I was not in a position to order from apple, so the 4GB ram upgrade for the air was not a possibility. After messing around with both machines for a good amount of time, I ended up getting the MBP. Let me give you my reasons:
-Upgradable: RAM (up to 16GB) and HD. These were huge incentives for me. While 4GB of RAM today is sufficient, 3 years from now I feel like I would be in trouble. For me, the MBP was a lot more future proof
- HD space. The 128 GB hard drive just seemed way to small for what I wanted to do. I felt like everything I installed had to be checked for HD space. I just didnt want to worry about iMovie projects, mail, photos, itunes files, etc.. taking up too much space. Like I said, this was my first mac and I wanted to be able to do just about everything on it
-Battery life: The standby time on the Air is really nice, but I just dont feel like the actual usage battery life is as good. (I know CNET says otherwise, just my experience)
-Screen: I plan to run an external monitor at home so the 1280x800 res didnt bother me. Besides, I felt like the MBP screen was beautiful color wise. The MBA screen seemed a little washed out, or florescent. The glare on my MBP doesn't bother me since I prefer to work on my computer indoors.
--Speed: while the SSD in the Air allows it the nearly match the MBP in current benchmarks, I have the ability to upgrade to an SSD at a future point. With this upgrade, the MBP will fly.
-Cost: I received a student discount. The discount for the MBP was greater than the MBA. With the Air, I needed a superdrive and external HD right away. That added an additional $180 to my purchase plus the $100 difference in student discount at BB left a price difference of $280 for me. This was a major incentive.
-Durability: the MBP just seemed more durable, especially the screen.
Anyways, these were my main points for getting the MBP. The Air is a great computer, but IMO is meant to play a supportive role to a desktop machine. If had had an iMac or a 15" MBP I would have certainly gone with the air. The weight of the air is incredible and that 1.5 lbs is very noticeable, but in the end, that difference wasnt large enough to overcome all the advantages of the MBP for me. Anyways, hope this helps. Good luck!