I'm basically in the same boat.
I need a new (lighter) laptop for my 6 month trip to Tokyo to write my Master thesis there.
I could get the top 13" Retina for around 160CHF (175$) more than the Air (256GB, 8GB RAM).
But I don't know if it is worth it.
I want more screen real estate compared to my current 13 MBP, because I don't know if I will have an external monitor there (or if I buy me a cheap one, which I really consider if the lab does not have a spare one).
The 1440 of the Air are already quite an upgrade when running two office windows side-by-side, so would the 1680 give me even more (or would the text be to little for a whole day of writing and reading)?
And I am a little bit worried about running the Retina at 1680 performance wise.