My high-spec 2012 MBA is worth somewhere between half and two-thirds of what I paid for it a year later. My wife's base MBA, on the other hand, wouldn't fetch half of the original purchase price today.
There are 2 ways of calculating depreciation: as a percentage of original price, and as an absolute value.
While a high end model may lose less of its original price in fractional terms, it may still be a higher dollar amount simply because the price was higher to begin with. Generally speaking, any upgrades you add to a laptop
may increase the resale value, but will never "pay for themselves" in that regard.
The price difference between the two models you are considering is almost $500. I seriously doubt the resale price will differ by that much in a year or two when you decide sell.
Let's do some math, just because I'm bored
Suppose the rMBP loses 1/3 of its value: $1350 / 3 = $450
And the MBA loses 1/2 its value: $877 / 2 = $438
So you would lose $450 when you resell the rMBP and $438 when you resell the MBA. That's pretty close actually, but neither number is guaranteed as prices can fluctuate.
Then there is time value of money. Since the original price of the rMBP is $477 more, we can go all out and add interest lost from not having that money available for a year or two that you would be using the computer. Assuming a conservative 2% annual interest rate, that's about $10 per year. But I'm just being extra anal here
The moral of the story is that you should buy what fits your usage best and what you can afford right now. The difference in resale value is likely not significant enough to outweigh your direct requirements in a computer.
