I'd suggest that you get a MBP and an iPad; given your stated desire for portability, and apparent need for a computer with a proper keyboard, the iPad and MBP strikes me as being an excellent combination.
By all means get a MBA if portability is a key requirement. However, my personal view is that a MBA and an iPad together might be excessive, as the iPad alone would satisfy the need for something easily portable.
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depends what you do. One is a computer and one is mostly for reading/apps/games. You will not be able to be as productive using an ipad as you would a mac. Do you need a PC for work/school?
Very good post which sums up the difference between the two better than I did.