I have a MBA at work, but haven't used it for image editing yet. It probably depends on what you are doing. In many scenarios (viewing a lot of images, slight editing in Aperture or Lightroom), disk speed can be the limiting factor rather than processing speed. In those scenarios, the faster SSD can more than make up for the lesser processor speed compared to the MBP.
In the bast, I have been using an old white Macbook (2007) with Aperture, and it was okayish. That being said, a MBA should do ok, and you will miss the faster processor maybe only if you apply a lot of filters and adjustments, brushing them into the image, etc.
You might want to consider a MBP instead of the MBA if that allows you to leave the external disks at home. The 13'' MBP (which I use for personal use) is still a nice size for travel, and easier to pack than a MBA plus external drives