All things considered, low air pressure is more likely to cause pinch flats than a tube fitted to a small-but-still-within-spec tire. So I’d go larger in your case. It also means you might consider investing in a suitable air gauge, if you don’t already have one, so that checking air pressure regularly is a low-resistance activity.

Since you do have two bikes that can accept the same tubes, you can also view it as a reliability improvement: if either bike has a flat, you are more likely to have the spare tube on hand, meaning you can get back on the road quicker. So even if pinch flats become 0.0001% more likely, you’re ready for it.