Meanwhile, the base model will also take a price hit of significant size as well, and will be lower in overall resale value.
The added RAM will not rain your battery. Overall, it might EXTEND your battery life due to fewer page-ins and page-outs to the SSD. Granted, this used to be a bigger deal with hard drives, but, there is still power wasted when reading/writing rapidly to SSD, not to mention the additional wear placed on an SSD when doing this.
I would suspect that a savvy buyer of a used MBA would also ask to check the lifetime writes stats with trim-enabler or another tool, and factor that into their purchase.
Ultimately, it's your choice. When I buy computers, I "invest" differently than you do, it would seem. While your main concern seems to be resale value, my main concern is the output in terms of productivity, which is where I get my monetary and economic value out of a computer that I purchase. If I can store more on a system, and get more out of it in terms of heavy usage, then the extra money spent up front is money well spent.
So for me, more RAM equals better productivity, more tasks run with less frustration and delay, and better overall value.
Then again, I don't sell my computers right away. My "top line" work system is a 2012 iMac 27 inch, i7 with 24GB of RAM. And I have a 3 year old 2010 MBP that I'm replacing with a maxxed out 2013 macbook air. The new MBA will replace the MBP as my primary "on the go" computer, while the MBP will be demoted to being an at-home, leisure light-duty system to do things my iPad can't. Later, I'll get a new Haswell mac mini, and then I might sell/give away the MBP.
I will likely keep my MBA for two years. Maybe three.
You're beachballing with 16GB of RAM. To me, that would indicate that 4GB is a pretty hard step down. While you will get a huge improvement with the SSD, it can vary with use over time as well as how much of that capacity is taken up with data.
However, the speed-up gained by higher RAM is constant and consistent.
If you're struggling now with 16GB of RAM, I think you will struggle with 4GB... EVEN with the SSD and the better processor. Although you're not playing games, you appears to be doing some heavy lifting with your web development, by running MAMP and probably having multiple tabs open in web browsers that have their own tasks requiring RAM and CPU.
Any CPU bottlenecks you get will be alleviated in mavericks with App Nap, most likely. But RAM usage will still be an issue.
