1 1/2 Lbs vs...
The 1 1/2 lbs. is one advantage of the MBA. It's display is also brighter, and it happens to be gorgeous looking.
I'm coming from a 15" MBP. At 4.5 lbs the 13" is a noticeable weight loss. Hefting it in comparison to the 13" MBA I can notice the difference, but unless I'm hiking for days with a full back pack, 1 1/2 pounds just is not going to matter.
The 13" MBP with a 2.6 GHz processor and SSD would perform better. How much of that I would notice in real world use I'm not exactly sure. But if going from 1.8GHz to 2.1GHz in the MBA is worthwhile, then going from 2.1 in the MBA to 2.6 in the MBP is too, perhaps more so. I doubt I would upgrade to 8Gb RAM, but it would be nice to know that I have that choice and it may come in to play at resale time.
Battery life is a serious factor: MBA rated at 7hrs, MBP at 10hrs. THAT is huge. Missing Gigabit Eternet could be a real problem. Just last week I visited a client and the only way I could connect to the internet was by using their Ethernet hookup.
I miss the IR receiver that they left off of the MBA too. That means I can no longer use the Apple remote when I do presentations, and I have to carry another dongle (in addition to the usb ethernet one, AND the one for the projector.) My MBA is going to look like an octopus with all these things sticking out all over the place!
Back lit keyboard is a luxury I have really come to appreciate, and I do much prefer the look of the black glass bezel of the MBP. The metal around the MBA display looks dated to me.
Firewire, optical drive... may never use them. On the other hand, they might come in handy once in a blue moon, and then it would be very nice to have them.
This is a tough decision!