Join Date: Feb 2000
Location: Menlo Park, CA, USA
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Posts: 6,912
Not too sure which machines you are comparing here ("15" macbook air""), but the at this stage in apple laptop history there is no 15" macbook air, and if it is a 15" macbook pro, then it is going to be a retina machine only.. which is a HUGE difference in usability.
With the new low end rMBP 15" inch, yes the integrated graphics processor is nearly the same as the recent 13" MBA, but with the 16GB of RAM on the rMBP you'll have more headroom with the 15" rMBP for the memory the the intel iris pro graphics will use up at peak demand.
As others mentioned. IF portability is a priority, and IF a retina display is a preference I would really recommend the 13" MBP which today is probably one of the best values for the money of the mac lineup. It is a very very strong performer, better battery life than the 15", great screen and very fast. Can be configured up to 16GB RAM and with an i7 processor as well.