(EDIT: I'm eligible for an upgrade on AT&T, btw.) I'm debating whether to sell my iPhone 4, and buy an iPhone 4S, or buy the iPhone 4S, sell it, and buy something else like an iPad to be a companion to my iPhone 4. Either way, I'm sure a lot of you guys have done something like this, and I'm looking for advice. How should I do either of those? Where should I sell them, ect.?