Dedicated computer. I have a Garmin Edge 705 for cycling and a Garmin Forerunner 305 for walking. The battery life on both is about 12 hours or so. The battery on my cell phone could well die on a century ride (I'm slow) or on a multi-day ride over a weekend. Plus a device designed to do one thing well usually does that thing better than a multi-function device.

When my 705 dies (I've dropped it enough that it should be dead already), I'll probably go with something simpler like a Garmin 500. I seldom use the GPS mapping functions and if I do get lost, I'll pull the cell phone out of my back pocket and check out the maps on it.