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if you have a desktop computer you should go with 13" because it's easier to carry around and when you get home you can just use the desktop..

but if you don't have a desktop and Macbook pro will be the only computer you should go with 15".

Or just an external display at home

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I move around a lot everyday; perpetually in motion with my 15" mbp in my messenger bag, in one of the most humid countries on the equator (you only have to step out of the house to start sweating). After using the 13" mb white and 13" mbp, I found my love in the 15", perfect for my needs with regards to screen size and the weight doesn't bother me.
 
It doesn't make sense to buy a 13" MBP only because it's lighter than the 15". Those who want it light go for the MBA.

Lightness is a good thing, but in the end it all boils down to how much power you need. If the MBA doesn't cut it, you go 13" MBP, if the 13" MBP doesn't cut it, you go with the 15".

Now, if you're trying to decide between a 15" and a 17" like me you have a real problem... But even here weight doesn't matter that much. It all boils down to whether I want an eSata port or not :D. Still don't know how long it's gonna take for Thunderbolt drives to be available or how much they're gonna cost!
 
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