I prefer typing on the iPhone to the iPad- but I understand that when I upgrade to iOS 5 on the iPad I will be able to thumb type better.

I don't mind typing on the iPhone at all.

As for thickness, if you want that you can get it. There are third party cases that add extra battery life to the phone at the cost of thickness.
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-Jeff
Rome did not create a great empire by having meetings; they did it by killing all those who opposed them.