I voted bike computer, but I actually use my Garmin Forerunner watch since it tracks my heartrate and I also use it for all of my other activities. It can last for a full century ride with an active display/GPS/etc with some room to spare (unlike my phone).

I also use my phone, but I only have it in my pocket and I listen to the periodic verbal updates (from the Garmin app) and streaming radio (sometimes).