The 13" MBA will be pretty much *1/2* the weight of your 15" MBP. (2.9lbs vs. 5.6lbs) Don't let the smaller 11" fool you, the 13" is still a svelte little machine.
I bought an 11" coming from a 13" revB MBA, but after spending an afternoon hunched over it working, I returned it to the Apple Store. I found working on the 11" cramped, and everything on the screen was SO SMALL. Now contemplating skipping this revision all together in protest over the Blacklit keyboard omission.
But seriously, I think if you're taking the machine back and forth all day with you - you'll need the battery life (7+ hours) of the 13".
Yeah, the 13-inch seems like the practical choice. There's just something about the 11-inch though lol. I am going back to look at them today. The backlit keyboard omission doesn't bother me in the least. I was really excited when my MacBook Pro had that, but I haven't used it in the longest time.