My reason for going with a 15" MBP instead of an Air:
1: Air doesn't have any extension slots. I think 1 USB and 1 external monitor does it.
2: Air doesn't have a CD-ROM drive.
3: It looks girly
Dunno, I prefer the MBP on all fronts, tried out an Air for a week while my MBP was getting fixed, and the lack of slots on the thing really hurts it. In general I need 2 USB hubs to hook up all my gear, and not having a CD/DVD drive is a big bowl of suck if you need to install something. You can use another Mac in 'target' mode thru firewire to take over it's CD drive but that's a pretty damn ghetto solution if you ask me.