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Its a very hard decision between the two. I've even battled back and forth before picking up a 2.4Ghz 13" macbook pro (sold it off though, cant stand the glossy screen!).

11.6" vs. 13"
thin vs. less thin
1.4GHz vs. 2.4Ghz
Semi-glossy vs. Glossy

I am actually thinking about picking up a mba.

Don't forget non-backlit vs backlit keyboard
 
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