If either phone can shoot in true RAW then the samsung would be a better choice since it seems to have better dynamic range and overall sharpness, and and preferences for more vivid or more realistic color can be done in post.
Then again, most people who would take advantages of RAW probably have an actual camera better suited to the task.
The Samsung S9 offers the option to shoot in RAW within the camera app. An iPhone and a Pixel camera can shoot in RAW using a third party camera app.
For anyone who is interested in the audio part of the videos they take, I do recommend taking a set of earbuds to the store and recording a video and listening as well as watching it with your earbuds plugged in to the S9, which can record videos with very nice stereo audio. On my S7 Edge and S9+ the immersive stereo audio of the recordings makes you feel like you're right back in the scene. It adds a special element and life to watching old family videos. For some reason my S8+ didn't record with as much fidelity and clarity and neither did my husband's. But the S9+ has its stereo mojo back. However, there are stereo lightning mics available if you prefer iOS.
I've had an X and now an 8 Plus, a Pixel 2 and an S9+ and they each take the best picture in different circumstances. You could go bonkers comparing them. None of them gets all of the colors completely accurate in all the many varied lighting conditions you will encounter.
The weakness of iPhone cameras is they lose some nuances of light and how light reflects and plays off of subjects. iPhones will flatten all of that out of a picture somehow. But iPhones add warmth that plays well to viewers. iPhones are prone to blowing out highlights or losing much detail in highlights.
Pixel cameras somehow capture what light is doing and brings it to your attention. So that light shining in from a window and bouncing delicately off of various surfaces in the room is preserved. On an iPhone all of that light gets processed into a warm glow or just disappears altogether.
I haven't had the S9+ long enough to determine how it works with light. I do think it tends to let a bit too much light in on some scenes, leading to a bit of a washed out look in some photos. This is easily tweaked, however. I need more time and experience before I attempt to contribute anything constructive.
The X and 8 Plus and Pixel 2 and the S9+ work some minor miracles in poor lighting. We have excellent options!