Depends on what's most important to you -- weight & higher resolution screen, or performance and expandibility being the primary differences.
I'm extending the life of my late 2008 MacBook Unibody to 8GB RAM and a new hard drive. (More memory than I could have gotten at purchase.) Which will then get handed down to wife and kids when I buy the new 13" MBP
With the MBP you could also do a dual-drive setup, replacing the SuperDrive with a smaller (128MB?) SSD for the OS / Apps, keeping your documents on the 500 GB HHD.
Thanks for your comment. I have similar plans as yours; in fact, I have a black MacBook (early 2008) that I plan to upgrade with extra RAM and OWC 480GB SSD, and give it to my wife.
I just read a very nice article comparing 13'' MBP & MBA:
http://arstechnica.com/apple/2012/06/faceoff-13-macbook-pro-vs-13-macbook-air/
Something I learned is that MBP's display has better color gamut. Though I am not a graphics designer, I am picky with colors.
Recently I bought a Dell display and found myself using that external monitor most of the time. So I guess the extra resolution of MBA won't mean much to me.
By the way, I usually use my computer for writing papers, organizing papers via Papers, and organizing photos via Aperture. Occasionally I use iMovie or Photoshop. And for that matter, I think there wouldn't be much difference in the computing power between MBP or MBA.
It seems, at the end, for me, that the issue comes down to whether it is worth to spend extra ~$400 just for lighter weight. A difference of 1.5 pounds is significant, but there are a lot of nice things I can do with the saved $400.
Ah... one more thing, MBP has anti theft slot, but MBA doesn't, and I thinks this is important. Though the anti theft cable is not 100% secure, I find it very useful when I work in the library.
So I am leaning towards MBP; but since I am in no hurry, I would like to wait to see if there are other comments.