A new thought is that perhaps with iPhone or the iPad, the best device is the brightest. Therefore, a benefit of the new iPhone Pro to be brighter in the sun, and also for the bigger iPad Pro to be brighter.
Then perhaps comparing “nits“ is a good idea.
The iPhone is just a GPS device
At the end of the day, it seems that the most useful part of a smart phone or an iPhone is the GPS functionality, whether you use Google maps or Apple Maps to navigate. A big issue I have faced is when I am driving in the bright southern California sun, Suddenly the screen of my iPhone dims, and then I can no longer see the driving directions. Therefore then, perhaps a good financial investment is to buy an iPhone with the brightest screen, the newest iPhone pro, to simply ensure that you could still see your navigation, even in the bright sun.
The upside of spending more time outdoors
I do believe that for human physiology, to spend as much time outdoors, in the direct sun is maximally beneficial. For your mood, your strength, your happiness, etc.
Therefore, having devices with brighter screens in the bright light you are outdoors is a good idea.