I currently have a 2013 i7/8g/512gb MBA but previously had a 2011 i5/4g/256gb MBA (4gb and 256gb were maximum you could get in 2011). Really, I was very happy with that machine and I ran Final Cut Pro, Logic Pro, Photoshop, VectorWorks and other professional software with no problem.
If it were me, I would get the 256gb SSD because I want to keep as much as possible onboard. And if I'm editing video it's especially nice to have enough space for a project or two.
Others will point out that the SSD is user upgradable but RAM is not. This is very true, however personally I don't think an SSD upgrade makes sense financially unless prices have dropped a lot, and I'm not sure if anyone even offers one for the 2013 models yet.
For me, 128gb just wouldn't be enough for what I want to keep on the laptop so that would be the deciding factor. But you need to decide that for yourself.