I have both and echo the sentiments. I don't always have my iPad with me, so iPhone is great for quick on the fly portability.
I use apps on both, Kindle is maybe the one I use the most in common, but the two also have different uses-like I would never do PDF annotations on my iPhone (even though I can). Games I use on both, but most of my main reading is on iPad, although I like having books on iPhone too just to always have on hand in a pinch. I take notes on iPad, something I don't do on iPhone, except a quick list in the Use Your Handwriting app. I do quick look-ups on iPhone if I'm out, but never use it to surf the internet if I'm at home-iPad takes the cake there hands down.
In other words-having both is not a duplicate in the way people might think.