I have both and don't always take iPad on the go, because of its size. I take my phone everywhere, and of course I still need a phone.
iPhone was my gateway drug to iPad. I fell hard for my first iPhone because of all it could do, but it's puny compared to iPad and isn't nearly as navigable for many things.
iPad is much better for Web surfing and navigation, for writing emails and notes (though I still use a laptop or PC for real writing, even though I have an Apple wireless keyboard for iPad). I use iPad for many other things, including news, Netflix, this forum, Skype and FaceTime, banking and investments, travel and travel planning, shopping, reading and signing documents from work, doing the occasional work presentation. I don't play games.
Reading is my primary pastime, and I read books on iPad for about 1.5 years, till I got my Kindle Fire, which is a more convenient size for me for books, especially on the go. I prefer its weight and size for hours of reading.