> Sure, I don't have a Hi-Res screen (the one Pro feature I wanted on the 13"), but I can actually upgrade my computer in 3 years when 4GB RAM becomes a bottleneck. Now that's a low year-over-year computing cost.

You can also upgrade your MBP RAM cheaply today (or tomorrow) to 8GB.

Not only does the MB Air not support 8GB, it's soldered on, so you can't upgrade.