Well, mate, you know best. If the MBA is fast enough for you, I'd say it's the best choice. Sell both of them, and get the $1800 MBA.
If you feel you sometimes need more power, trade both of them for a 13" MBP. Based on your needs, go for the 2.26 one or the 2.53. I'd recommend the 2.26 one, the processing power should be enough, and if you upgrade it to 4GB/RAM and a 500GB HDD, you're getting a Mac with a bigger HDD and the same computing power for the same price.
If you feel you sometimes need more power, trade both of them for a 13" MBP. Based on your needs, go for the 2.26 one or the 2.53. I'd recommend the 2.26 one, the processing power should be enough, and if you upgrade it to 4GB/RAM and a 500GB HDD, you're getting a Mac with a bigger HDD and the same computing power for the same price.