Which lasts longer? I'd assume the 7/8 with an apple smart battery case, but can't find any info online. I'm going to buy it for my daughter. XR is not in the discussion.
One thing to keep in mind is that while the Plus version has a larger battery, it also has a larger screen with a higher PPI, so at least some more of that battery gets used up on that in comparison to what gets used up on a non-Plus version.
I haven't quite compared the exact numbers, but I think what a battery case adds as far as battery capacity might also be more than what the Plus battery has over the non-Plus one.