True, they're a bit tricky to compare, but I don't think the point on ram/storage is super important in this particular case.
1) OP wants a device for 'basic computing and to write'. That tells me Facebook, youtube, Netflix, Google, Email, Skype, Word etc.
8gb / 256gb isn't going to be really important for that.
Imagine the MB had 5 million RAM and Storage, and the MBA 4gb / 128gb. I don't think it'd make a big difference for this particular OP.
2) If you do want more performance, then sure you get more memory on the base model, but the CPU is a bunch slower, and it needs to push more pixels. So to say that a MBA needs upgrades for it to perform as well as the MB probably isn't true, it's quite likely the base model MBA will perform better than the MB overall.
3) Even if you want extra RAM (again doesn't look necessary at all for his particular use), it's $100 more. You're still paying $200 less, while getting 2 hours more battery, better CPU/GPU, 1.3" more screen and more travel/space on your keyboard.
4) Storage upgrade, the least important for him it seems, is unnecessary. But if he does need storage, a nifty device or external HD are much cheaper.
I think the base model MBA is more than sufficient for OP, if he needs more then (3) MBA with +4gb upgrade, is the most realistic option which is still $200 cheaper.
I think you'd have to pit that against having a retina screen and a slightly thinner/lighter device. I'd personally prefer an HD 13.3" over a 12" Retina because anything below 13" is just too small for me. And if I then get $200, a better CPU/GPU, better battery and keyboard and better ports, I'm happy.
Looking forward to a possible "14 MB retina that's the same size as the 13.3". That'd be awesome.