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Each has its niche.

My MBP is my content creation & editing device. The iPad is my content consumption and crosswords/gaming device. The iPhone 4 is my on-the-go video recorder, digital camera, photo editor, ebook, audiobook, everything else unit.

I love the resolution on the iPhone 4, but if I'm curled up on the couch, I'm using the iPad. Curling up to read before bed though, I actually swap back to iPhone 4 again because it's smaller and lighter, and less screen glow.
 
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