I have both an iMac and a MBA, so maybe I can help.
Personally, I love both my Apple computers and wouldn't want to give any of them up. I use my MBA for University everyday. When I'm at home though, I always use my iMac. The screen is terrific, although it is super glossy (if that bothers you; personally I like glossy screens). The wireless keyboard and magic mouse (or trackpad if you pick that) are great, and I feel that Apple really did a good job with the gestures on the mouse; its the first machine of its class to have such a feature. Also, the iMac is more powerful; quad-core i5 with a dedicated GPU is a lot better than the MBAs dual core and HD 3000. This is a really big upgrade if you ever play any games, or do intensive work.
Overall, if your only using your MBA at home, I'd recommend the iMac. You should be able to get 80-90% of your money back on the MBA, assuming there is no damage. And because the iMac is cheaper than the 13" MBA, hopefully that should be enough to buy it. Just remember that theres no going back, and once you get rid of the portability of the MBA...well good luck carrying your iMac about with you

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