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if your buying a 27.5" I'd go the i7 2.8GHz, in my opinion it's the best value.
Dual core, you have two processing cores, with the i5 or i7 you have four and with the HT (Hyper threading) from an i5 or i7 (not with dual core though) that makes eight cores, much better value.
With TurboBoost and the 2.66GHz i5 it boosts up to 3.2GHz when all cores aren't active (not quite sure how it works exactly). With the i7 it goes up to 3.46GHz.

So technically you're paying slightly more to get six more cores, that run faster than the dual core @ 3.06GHz when not all in use?
 
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