Remember that you can configure the S9+ to allow swipe down on the screen to lower the notifications, making one handed use easier, or you can do the same with the Nova launcher. There's also the triple click on the home button which shrinks the whole screen (although a bit too much for my liking) for occasional one handed use.
In terms of differences, I've found for my use profile (I use a lot of apps simultaneously), the 6GB RAM in the Note 8 makes a big difference over the 4GB I had in the S8+. I also love the bigger screen of both the Note 8 and S8+ for content consumption. I watch most of my content on my phone from YouTube to Netflix.
Depends a lot on what you plan to use it for. I wouldn't be using an iPhone with an LCD screen as my primary content consumption device because of the LCD screen, and certainly not the regular sized 6, whose screen is very small. But if you don't plan on using that it doesn't make a difference.
If all if comes down to is the 2X camera, I'd say go with the S9 because you may use that camera (I do), but rarely. And you shouldn't buy a phone based on a very rare use case if that use case makes every day use uncomfortable.