Phones are hard to see in bright sunlight. Bike GPS computers are waterproof, viewable in full sun, and the battery lasts a long time.

Phones are way better at browsing around the map. I have the very low cost osmand+ app. It downloads maps in advance, so no cell signal is needed, and the maps display very fast. You can try out the free version, with a limited number of map downloads.
osmand: free, a limited number of state or country downloads.

I reviewed osmand+ in this post. It's $9 for an unlimited amount of map downloads. Contour lines are an additional plugin.

Now, the newer version of osmand has an icon to switch between driving, bicycle, or walking, with different views shown.