My experience with using smartphones was battery drain if using them with the display on. My iPhone lasted a bit past 2 hrs. I switched to Garmin devices so haven't tried my Galaxy S4 with any cyclometer/nav apps yet. Its AMOLED display uses less power the fewer the pixels illuminated as opposed to the constant draw of the backlit LCD type of displays on iPhones, so conceivably could last longer.

A good site for detailed technical info/reviews of BTLE/ANT+ cycling sensors etc. is http://www.dcrainmaker.com/