ditto
I had a 13" pro, sold it to get an ipad, then sold that and decided to wait and probably get a pro again...until the Air came out. I went to the store ready to justify the 11" (my fav computer ever was the Powebook 12"). While it wasn't in any way "disappointing", I left with the 13" Air. The main reasons were:
Bigger screen, handled photo processing better (I took an SDHC card with me, the apple guy was happy to let me use it), practically just as light and portable.
The processor made a difference, it was, to me, noticeably better at even just loading (including thumbnail creation and all of that) photos than the 11". I think I could have just dealt with that, but it was obvious the screen size would have made photo editing a pain. Also, the 13" has a better (the last gen 15" screen I believe) screen in general, including wider color gamut, and I could tell. Even then, I was trying to make it work, but what pushed me over the edge was the added SSD capacity. You just get a lot more for the extra money.
Those are my thoughts on the 13" compared to the 11".
As far as whether you should keep your MBP, I would suggest it depends on how often/far you carry it around on your person. If you have to be mobile and need your computer with you, if you bike around with it on your back a lot, and/or if you're a student, there's no question: get the 13" Air. If your computer lies around the house or you drive most everywhere, keep the MBP and save money. I LOVED my MBP. I just couldn't carry it around on my back all day any more, along with a bunch of books. Hope that helps!
p.s. Keep in mind i REALLy wanted to justify getting the 11". It was the 12" powerbook I had been waiting for. Having said that, from day 1 I've been supremely happy with my choice, and feel I would have regretted the 11".