You should know that Apple is strongly recommending against developers doing anything different on screen for the different sizes. Besides maybe fitting a tiny bit more text, more like half a line, layouts and behaviors/expectations are not *required* to be identical but very strongly recommended.
The reality is, you probably won't notice any difference unless you're looking at a picture under a magnifying glass.
Apple has gone to great lengths to try to restrict the decision here to one of style and fit, not function. This is probably why they don't go into any meaningful detail about how much more battery life the 42mm version will have even though they do recognize it.
So the advice I would give you, mr. complete stranger on the internet

is to just follow your sense of fit and style and not worry about the few extra pixels on the screen.