I've got both an iPad and an iPhone. I use the iPad to plan my flight, review NOTAMs and whatnot, and when I'm happy with the plan I'll transfer it via de SD Cloud to the iPhone. That becomes my primary GPS during the flight.
I also keep a paper map handy with the same line drawn on it as in SD, and I keep the most pertinent information from the plog on a paper backup. Between thse bits of paper and the iPhone I have more than enough information to complete the flight.
In a pinch I can plan the whole flight on my iPhone as well but the smaller screen size makes that a lot harder than on the iPad.