Read this Anandtech article on the subject.
http://www.anandtech.com/show/7113/2013-macbook-air-core-i5-4250u-vs-core-i7-4650u
I would go with the i7. Negligible effect on battery life but significantly faster and more future proof with better resale value.
People don't realize how much different the bigger cache in the i7 makes. It's not just higher clock speed. Combined its up to a 30% increase.
Personally I think 8 GB is a must have especially since the gpu eats up 1 GB all by itself.
To me the sweet spot is 8 GB ram, 1.7 ghz proc, 256 GB SSD unless you just have to have 512GB ssd.
Also the 256GB ssd is faster than the 128GB sad but you probably won't really notice it.
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No difference except paying tax in some states.
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Yeah that's wrong and bad advice to boot.
