I think it is just brighter backlight on GPS, they were made for outdoor use (more rugged, waterproof, no need to buy a seperate case for protection, etc.) and phones are generally designed for indoor use. My phone in direct sun is almost impossible to see. But my phone cost $105 USD, so maybe your phone has a more expensive backlight if you can see it with the sun shining directly on it.

The larger screen on a larger phone or small tablet I think is better for planning a route. I usually bring a 7 inch tablet to use in the campsite for route planning, etc. The dedicated GPS units have much smaller screens that are best for figuring out where the next turn is, if it is close.