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If you just want to charge your iPhone 4, get the cheaper one. I think the only difference is that the more expensive outputs more electricity to charge the iPad, since it requires MOAR POWER.
 
So I ended up buying the 2.1 amp version. A few reviews I read online and the Best Buy sales guy stated that the 1 amp version doesn't work with the iPhone 4. Plus the 2.1 was on sale, and came with a charge/sync cable. The included cable has the largest dock connector plug ever, and doesn't fit into any of my cases, so that goes in the drawer. I have to say I am very impressed with the 2.1 amp charger, it's tiny and it charges the iPhone very fast.
 
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