I can speak for myself only. I am on a 13" cMBP that I picked up this spring after my previous machine died leaving me in a position where I needed a laptop on a tight budget.
The choice for me was relatively easy, as the combination of price, user upgradability, ports, and IR receiver gave me everything I needed plus the potential to have what I wanted. The only real loss was the retina display. It'll be a while before I miss the ac wireless.
After being perfectly content wit the resolution and pixel density on my 2008 MB, I knew I would be fine without the retina display. I haven't been spoiled by it yet, and it will be several more years before I am.
As far as the ram, I can say that 4gb will be fine for your uses. 4gb is fine for my uses, unless i need to run a virtual machine, but then the biggest limitation was my old spinning hdd (unbelievable what an ssd will do in this case). With some heavier multitasking, you will find that you start paging out a little, but it won't be show stopping on either the Air or the MBPr. Going to 8gb does give you some flexibility now and in the future, especially if you want to run a virtual machine for windows, however it will be overkill for you normal usage. After swapping out one of my 2gb ram sticks for an 8gb ram stick, I can run a windows vm in virtualbox with 4gb ram dedicated and my ram pressure only hits about 55%.
While the base models are fully capable, I would recommend, if budget allows bumping to 256GB of storage, if you like to keep around a sizable chunk of media, or you are required to work with sizable data sets that you want to take on the go. Next, bump the ram to 8gb, but only if you feel confident your usage may change and/or you plan to keep the machine for a long period of time.
Good luck in your choice. Either machine will be spectacular, and serve you well. As a final statement, you can't miss what you don't have
