Re: Time for a new MACbook but, specifically, which one???
Performance difference between 11'' Macbook Air, 13'' Macbook Air, and 13'' rMBP is not substantial. All three entry level CPUs should be adequate for you unless you do significant photo editing/rendering in which case you will want to consider the rMBP with a CPU upgrade. Once you pay for the CPU upgrade in the rMBP it becomes a significantly better computer than the MBA.
I recommend purchasing a minimum of 8 GB of RAM. It will add $100 to the cost of the machine but is a smart buy to extend the life of your Macbook. If you used your previous machine for 6 years, than 8 GB will go a long way in letting your new computer last that long as well.
Impossible to recommend a local storage size without knowing how much you plan to store locally. 128 GB is more than adequate for me but I also run a Mac Mini that I have modded aftermarket with additional storage. If you wish to save money, I would personally recommend just sticking with 128 GB of storage and considering an external drive down the road. They are not particularly expensive and you can get a fairly sizable plate drive that is good for data storage quite easily.
IMO pick the screen size you want and go for the rMBP if you like the idea of a retina display (though I think the 13'' MBA's 1440x900 resolution is quite good already). Do keep in mind that the 11'' MBA has the worst display resolution of the MacBooks. The good news is that 768p is not completely terrible on an 11'' screen.