I find myself using both devices in very different ways — for example I'll listen to an audio book on my iPhone but read an eBook on my iPad. I realize that both formats are "mobile devices" but in many ways they each feel like their own category in terms of usability. I also notice that I use the iPhone for very quick tasks while I'm standing and on the go (i.e. multitasking) — while the iPad is best if you want to ignore everything else around you.