Although I don't have one, I'm still going to throw you my view point. It has been my experience that the large advantage is in the new CPU and GPU. In order to watch smooth videos from web or other apps, without dimensioning performance (your battery is already shot though), the newer chipsets makes things smoother. For instance, my MBA 2010 lags quite a bit when loading any video it seems. I think you would feel the difference with the rMB.
However, I would not bank on the rMacBook lasting seven years performance wise.... Your usage is minimal, but I can't imagine seven years of usage. It's worth noting that the rMBP is extremely sleek - have you looked at one in person? That machine would last your usage seven years much better than a MacBook.
I think the MacBook is awesome for portability (which enhances usability for me), so much so that after reading these forums, I'm rather confident that I'll be going MacBook for my next primary laptop... But I will plan to upgrade near annually while cycling machines to other employees (those in less demanding computer roles) and family members. If I were in a different place, where I was buying for even three years in the future, I would buy rMBP -- but my use case is different...
It all depends on what portability means to you, in my opinion.