I am not an expert on this, so hopefully someone can correct me if I'm wrong.
One thing that people used to complain the most about rMBP 13" last year was its 128 gb. So I know that if you are going with 128 gb, you might consider getting an external hard drive in the future. If you are not into videos or photography that usually take a lot of space, 128 gb might be just fine.
For RAM, there is a big difference between 2 gb and 4 gb, but between 4 gb and 8 gb, I am not quite sure. 8 gb is one of the most common. If you do a lot of multi-tasking and surfing the web, 8 gb will be better.
For 1.3 GHz vs. 1.7 GHz, a general consensus seems to be it is worth extra money for 1.7 GHz. Each 1.7 GHz core runs 10-30% faster, and some people usually say it is totally worthwhile.
If you have a local Apple store, I would suggest you to go and check it out in person. Sometimes, just talking in general might not give you the "feel" of which MBA version is better.
P.S. Have you looked into "like new" rMBP 15" in Amazon? Even the old versions like mid 2012 and early 2013 are not too different from the most current one, and you can get rMBP 15", 2.3 GHz quad, 8 GB RAM and 256 GB for ~ $1500. I don't know about other guys, but if I were you, I would have taken this rather than getting a brand new MBA for $300-500 cheaper.
http://www.amazon.com/gp/offer-listing/B007471PZQ/ref=dp_olp_used?ie=UTF8&condition=used