To be honest I don't see the point of an ultraportable having super powerful processors, which is why I think the MacBook Air's are fine with the Core 2 Duo processor's that they currently have. An ultraportable is supposed to be a thin and light computer for doing light work on, such as e-mail, web browsing, watching videos, typing up a report, and the like. An ultraportable is not for doing intense video editing and rendering work or doing other processor intensive things.
In my eyes, the MacBook Air outperforms the Series 9, and also, the MacBook Air is thinner. The Series 9 is .64" thick while the MacBook Air is .68" thin and tapers down to .11". The tapering on the MacBook Air makes it look thinner in comparison to the Series 9, and it feels better in the hand, too.
Now, onto the IGP that comes with the Sandy Bridge processors in the Series 9. Well, in comparison to the NVidia 320m that comes with the Core 2 Duo chip in the MacBook Air, the IGP sucks. Plain and simple. It's been shown to be 50% slower than the 320m.
Another thing that the MacBook Air has over the Series 9 is the resolution. The 13" MacBook Air has a resolution of 1440x900, while the 13" Series 9 has a resolution of 1366x768, which is significantly lower than the resolution on a MacBook Air, especially when the 11" MacBook Air has the same resolution as the 13" Series 9.
Also, I can't forget to mention the fat that the Series 9 is running Windows OS which in my opinion is way behind Mac OS, but again, this is my opinion.
Lastly, the price. The price, the price, the price. The 13" MacBook Air can be had $1299 for the base model, while the base model of the Series 9 is $1649; significantly more than the price of the MacBook Air, especially when you take into consideration some of the things that I have listed above, and yes, while some things I listed above can also be seen as negatives (for example, I know some people will disagree with me about preferring the Core 2 Duo over the Sandy Bridge processors), most of what I listed are positives.
So, in my opinion, the MBA is much better than the Series 9 for all reasons stated above.
Again, this is my opinion and people may see things differently.