The 17" screen to the 13" or 11" is a huge trade off... The entire package, including sound, screen, peripherals, and connections are going to be much superior in the 17". The only reason to get an AIR is portability - I used to have a macbook (for portability), but after seeing it sitting on my desk most of the time, I've sold it and switch to an MBP. You get less heat, quieter work (fans don't kick in like all the time), and overall a much better screan to look at...
Now - I'm never home these days, so end up dragging a netbook along... so I just sold the MBP and ordered an AIR... for portability... but will deffinetly miss the MBP...