Daylight videos from both phones come out well exposed and with high contrast. The iPhone's dynamic range in videos is wider, leaving more color info in the shadows, which the Galaxy clips to black earlier.
A particularly noticeable trait in the iPhone's videos is the actual presence of color - unlike the stills, the X's videos exhibit a very pleasing balance between vividness and reality. That's not saying the Galaxy's colors are bad, it's just that the we're happy to see the iPhone come to life in video.
As a general rule the iPhone's footage looks a lot more natural and unprocessed, whereas the Galaxy's videos have a relatively more sharpened look.
The difference between the two phones' 4K output quality from the main cameras is rather noticeable and the clear winner is the iPhone X, in both 30 and 60fps. Wider dynamic range, superior definition of detail without excessive sharpening, better color preservation in the shadows - there's a lot to like about the iPhone 2160p capture. The Galaxy is not far behind, but it's not quite there either.
The line is blurrier with the telephoto cameras where S9+ is mostly on par with the iPhone in all but 4K/60fps. The iPhone wins hands down in this top mode, where the Galaxy's footage is uncharacteristically mushy.
You'll notice that the above videos weren't taken at the same time and we've used the samples from the respective reviews. We've had less than ideal weather for the past week, and the little sun we've seen was coupled with strong winds. We did shoot side-by-side at the same time, but those samples aren't strictly worthy of publication. Instead, we grabbed a frame of each for you to compare if pixel-peeping is your thing.
Both phones can stabilize video all the way up to 2160p/30fps in both H.265 and H.264 codecs. The Galaxy S9+ will not digitally stabilize 2160p/60fps footage at all, while the iPhone will.
We did a walk in the park with the phones side by side, and we can confirm that both will do a fine job of ironing out global shaking produced by walking (more like marching, this guy). That said, the iPhone exhibits a weird pulsation of sorts that we don't see on the Galaxy - so it's got to be the algorithm and not the walk that's doing it.
And here's the same routine only in 1080p.
Samsung's marketing machine is keen on making us believe 960fps super slow-mo videos are a killer feature, and we have to agree that they do make for some spectacular clips. The iPhone X has no answer to that - sure, it can do 1080p/240fps, but so can the Galaxy S9+.
That being said, in 960fps super slow-mo mode the S9+ is locked in 720p resolution and the slow-mo recording can only take place for a fleeting 0.2s (turned into a 6-second clip when played at 30fps). The auto detection of motion does help immensely to capture the right moment.