I use an Iphone 5s with a Wahoo Tickr bluetooth heartrate and Blue SC cadence/speed sensor. I use the MapMyRide app but it will work with Strava and many others as well. I have a Topeak Ride case II mounted to my stem so I can watch live as necessary. The phone on a full charge and a medium screen brightness will last about 3 hours if you leave the screen on constantly. I usually only look at it occasionally so battery life is never an issue unless I forget to charge it before riding. It also calls out average speed and distance over a settable period even when the screen is off.