I have a first gen MBA, and a unibody 15" MBP. I always carried the MBA because of the lighter weight, and the ability to slip it inside my portfolio that I carry to meetings (leather folder with 8.5x11 paper).
That was... until I got a 13" MBP. That has proven to be the happy medium for me. Yes, the MBA is notably lighter, but it's not really that much thinner. To me, the entire case might as well be as thick as the thickest part. I like having the extra memory in the MBP, the extra ports, the SD card slot, the optical drive (although I don't use it much), and I love the new track pads.
Both the old MBA and thew newer 15" MBP get much less use now that I have the 13" MBP. I'll admit I do like the less glossy screen on the MBA, though.
Even if I had the newer MBA with the 2.13GHz processor, and 128GB SSD, I think I'd still prefer the 13" MBP. I have the 2.53, with 4GB RAM and a 128GB SSD in it. The extra weight doesn't seem to make a difference to me, and I like having all the extra capabilities.
Still, I'm sure I'll be tempted by the next version of the MBA if the specs are decent. Then the competition between models will begin again.