Yeah, I think you're right. As much as it pains me to do because it looks so unbelievable, I think I might have to pass on the S8 and end up getting the iPhone 7 Plus. Portrait mode is pretty sharp too.
You do bring up a good point about recording video. I am going to see U2's Joshua Tree Anniversary Tour in 6 weeks and it would be great to have the best video recording options. Then again, I probably should just keep my phone in my pocket and actually watch the show. 😉
Aaaaghhh decisions decisions.
If you do go with an iPhone 7 Plus, this lightning mic will help you get better audio to go with your video:
Shure-MV88
It's small enough to not draw too much attention to yourself and does come with a case and a Tribble (wind cover) for outdoor recording. It was recommended to me by a moderator on this site and it did a very nice job recording my daughter's school musical. There are settings that enable you to cut out the recording of noise that might come from the sides from seatmates and so forth.
Regarding S8 vs Iphone video recording comparisons:
So far I've noticed that when panning, my iPhone 7 Plus video is smooth as silk no matter what setting I've got it at...1080p, 4K, it always pans flawlessly so far.
I can't say for sure with the S8 since I don't have it and only got to test at the store. But with both of my S7's and my HtC 10, panning can result in little micro-shudders. I'm not sure what the actual term is for it, but there's a kind of distortion as the camera is panning. This is one reason why I opted for my iPhone 7 Plus to record the musical even though my Samsungs and my HTC 10 have the ability to record in very immersive stereo audio with their built in mics.
Where I had some trouble with my iPhone was in getting the proper exposure of events on stage from where I was sitting. Unfortunately, during my daughter's biggest solo and some of her scenes, because she was the lead and onstage by herself, the camera refused to expose properly and totally washed her out. When other characters joined her in the scene, the camera was able to expose for all of the scene better.
I did an earlier concert recording of the kids at a different stage when using my SamsungS7 Edge and HtC 10. The Edge had no trouble with exposures or audio. The HTC 10 struggled with exposure under the same lighting conditions as the Samsung was operating under.
So do try and do a test recording with the S8 at the store and see if there are any issues with panning. Unfortunately there is no easy way to test for exposure problems in advance because the lighting at concerts and plays pose unique challenges that store and home and outdoor lighting do not.