I carry a spare phone mostly for apps. First an older iPhone 4s, my backup phone on a barebones talk/text/data plan for emergencies.
This week I paid $10 for a ZTE Majesty Pro on the Simple Mobile prepaid service. Even if I decide not to renew the service the phone will still work with GPS apps and wifi when I get home.
No worries about the battery running out. I don't run the display active most rides -- usually the phone sits in a jersey pocket or handlebar bag. So it'll last all day and night on a long ride.