In the long-run, you're much better off with 4GB of RAM in your investment, as it will definitely prolong the feasible life and usability of the MacBook Air.
Over time, OS X and applications you use will require/demand more system resources. Since you're not able to upgrade the RAM to 4GB in the future, you must make the decision of 2GB or 4GB of RAM now.
If you plan to use the MBA for a year, then resell it, perhaps you would be better off with 2GB of RAM. However, there is also the resale value, and the future use of the MBA may require 4GB of RAM for the MBA to be useful.
If you plan to use the MBA as your primary computer, or you plan to use it longer than a year, I would strongly advise you to upgrade to 4GB of RAM. There is no downside to upgrading other than a 10% increase (or $99) in the initial cost of the MBA. While there are many advantages to upgrading to 4GB of RAM. It will make the MBA much more capable, faster, and useful over the MBA's useful lifespan.
I strongly advise you to spend the $99 for the upgrade, as it will expand the usability of the MBA now and in the future. It will allow the MBA to run more apps, run apps better, run the OS better, and capable of handling growing demand for RAM over time when upgrading apps/OS X or adding more apps to the MBA, and allow the MBA to run multiple OSes simultaneously via a virtual machine; with 2GB of RAM, the MBA will not efficiently run multiple OSes simultaneously using a VM (Parallels, Fusion, or Virtual Box).
Good luck with whichever path you choose.