Tough call. They're both pretty sweet phones.
In my experience, 4G is nice and fast, but it drains the ever lovin hell out of your battery, and 4G coverage is still very spotty. That wouldn't be one of my deciding factors.
I'd go with the iPhone if it were me. Mainly because that's what I've had for some time. I'd probably recommend the Samsung for you, for the same reason. The Galaxy S is a kick ass phone.
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