HDR. Samsung has buried the toggle in the settings, making reviewers' lives miserable and users' less aware of what's happening with their photos. Anyway, it's what it is.
Both the iPhone 8 Plus and the Galaxy Note8 have three positions in their HDR settings - Auto, Off and On. The difference is that when the iPhone is in Auto, and it decides that the scene requires HDR, it'll go ahead and apply it at full blast, just as if you had turned it On.
Not so with the Note8. HDR Auto on Samsungs isn't as strong as HDR On, but then again, Samsungs do give you a live preview in the viewfinder, so you know what you're getting before even taking the shot.
On the flipside, the iPhone can be set to take a regular photo alongside the HDR shot, so you can choose the keeper later on.
The two phones' HDR algorithms are configured somewhat differently. While the Galaxy will salvage some of the highlights, it's mostly in the shadows that you're likely to see the biggest difference. The iPhone, on the other hand, tends to be more highlight-friendly and will preserve detail in clouds better.
In any case, we'd be happy leaving the phones in HDR Auto mode all the time. That's what Samsung wants you to do anyway, and on the iPhone you get the benefit of having both the HDR and non-HDR shot.
In low light, with the primary camera, the Galaxy Note8 has a distinct edge, be it thanks to the larger sensor or more mature noise suppression, or likely both. The Note's photos are sharper, with more detail and, at the same time, less noise. The Samsung phone also tends to retain color better, while the iPhone's already more muted hues lose saturation even further.
And here are side-by-side close-ups of the lower center portion of the above images.
It's pretty much the same when using the telephoto camera. Though, strictly speaking, below a certain light threshold both phones just digitally zoom the center portion of the primary camera to match the field of view you'd otherwise get when using the tele shooter - apparently, that way you get better images than with a smaller sensor and dimmer lens.
It should come as no surprise then that the Note8 retains its lead when zoomed in: sharper photos, better detail, less noise, more color saturation. That said, you don't want to be looking at either phones' telephoto camera output from up close - they don't hold up well to pixel-level scrutiny.
If you have a tripod handy you can tap into the Note8's Pro mode and opt for a long exposure/low ISO combination and get even better image quality with stationary subjects, or play a little with light trails. No such option on the iPhone, sadly, at least not with the stock camera app.