Either MBA will feel significantly lighter and be a much nicer mobile experience than any MBP.
While the weight an size numbers may seem small consider that even a 13" MBP is on average twice the thickness and 50% heavier than the 13" MBA. The difference is even greater for the 11. As others have said, when comparing the 11" and 13" MBAs, the size is more significant than the weight but percentage wise the difference is still significant.
Your question about the power supply really brings up the more important point of total system weight. Once in the "air mindset", it's not just the savings in the weight of the laptop, but the smaller lighter accessories, smaller lighter bag, fewer things in the bag because it's smaller and lighter etc. It kind of builds on itself once you realize how light a load you can get away with and keep trimming things down.
When I switched from a 13" uMB to an 11" air, I didn't just save the initial couple of pounds. Over time i scaled everything down and went from a bag of stuff weighing 15 pounds to one weighting 5, with all the same capabilities.