I had this very dilemma last week when we went to buy my birthday present - MacBook Pro or Air?
I'm writing this right now from the Air and I have no regrets, especially on the wallet side. Here's how I came to my decision, or rather how the sales dude helped me decide.
He asked me what I was going to be using the computer for. I answered:
1) writing
2) work - which means it needs to be lightweight
3) photo editing
4) internet
He said I should go Air, which kind of surprised me, because of the photo editing. If you don't need the fancy screen or massive amounts of on board storage, Air is the best choice. Since I'm not doing any video editing or large scale graphics work, I won't need that kind of resolution or power or storage. Besides, we have a whole server system to keep all of our media, documents etc straight ("NAS", interjects the DH) so the amount of storage isn't really an option anymore.
All of my media consumption is via streaming, either from the net or our server. I haven't watched or used or bought a DVD in years. Besides that's what the PS3 is for - the random Blu-Ray that we already own. DVDs have gone the way of the CD in my lifestyle, and my old MBPro's DVD drive rarely got used.
So, in the end I bought the 13" Air and an hour later was home, writing away. It's lightning fast, even when photo editing, which was my only concern. It's so lightweight that I almost forgot it was on my arm yesterday

. In fact my poor iPad 2 barely gets used anymore. I'm thinking of selling it and getting the iPad mini.
On a side note, I feel like in this day and age we have to think differently about how we do our computing. Keeping all our docs/media in one place isn't really the done thing anymore. If my laptop was stolen, I'd be a bit sad, but not upset because all my documents and memories are backed up not only on the NAS but (apparently, and I haven't checked this out yet first hand) on the cloud. But that's a whole other conversation.
Good luck shopping!