I have both. I mostly bring just my iPhone to work. At home, I use both at the same time. iPhone for texting, the rare phone call, the Visual Timer app for grilling, reading e-mail, taking photos of what my dog/fish/tortoises/garden are doing right now, and then at the same time iPad for Words With Friends, reading forums, reading Zinio magazines, Flipbook, cooking from recipes on Evernote. I just read an article that, to paraphrase, was about "Do I need a computer _and_ an iPad?" and the answer was, think of the iPad as a second screen- you're doing computer "stuff," but multi-tasking. I realize that most of my iPhone/iPad use sounds frivolous. I admit it.
Now I'm about to travel with the iPad for the first time and I have been so satisfied with what I can do with it, as opposed to what I have to have a laptop for, I am considering not even taking the laptop with me.