I develop mobile apps for a living and own a Nexus 5, Nexus 7, iPhone 5, and iPad mini with Retina display and cellular networking. I also own a Garmin Edge 800 GPS-enabled bicycle computer.
None of the Android or iOS displays are as easy to read as the Garmin in direct sunlight. By default, the Nexus 7 display can't be dimmed enough to make it comfortable for reading at night, even when displaying white text on a black background. Apps like Night Mode, Screen Filter, etc. are supposed to give you more control over the brightness, but I haven't tried them. I haven't tried reading at night on the Nexus 5 yet. As much as I like the Nexus 7, I have to admit that its 7-inch screen often feels very cramped when compared to the iPad mini.
Battery life of the two Android devices lags behind their iOS counter-parts. The Nexus 5 appears to be especially power-hungry. None of the devices has the longevity of the Edge 800 if you run the GPS continually, however. Getting a GPS signal seems generally faster and more reliable with the Edge 800 than any of the iOS or Android devices.
All of the devices seem well-made and should be reasonably durable if a little care is taken. The buttons on my Nexus 5 feel pretty flimsy, but I haven't had a problem with them yet. Because the Nexus 7 is smaller and thicker than the iPad mini, it feels like it might be a bit more durable. Unlike the Garmin, I would not want to let any of the iOS or Android devices encounter rain. While many people seem to love the revised mount used by the Edge 800, I think it's a big step backward! With my Edge 705, the release lever on the mount tended to break and the device would become locked to the mount until you pried them lose with a screwdriver or pocket knife. The mount for the Edge 800 doesn't break, but it also doesn't hold the device as securely. I've almost lost the Garmin several times and I've heard quite a few stories from others who have lost or nearly lost a device thanks to the new mount!