Your resolution needs should be considered when deciding on the camera for the best functionality for your needs. The GoPro HERO 7 Black is about the best out there for the size package (but... GoPro is suspected to be preparing to release the next model in September if they follow their historic trends, so you may want to wait to see what it looks like if they do). Will you want 1080P @ 240 fps to see the pellets reasonably well in slo-mo? Does your phone do this resolution/speed combo? If you're talking strickly imaging quality, I would suspect the sensor in the Go-Pro is larger, thus generally better (larger sensor means larger individual pixel light-gathering elements, and this generally translates to better image quality). The last thing to consider, and this is where I, as a past military professional photographer, and research tech for Eastman Kodak as digital photography replaced film-based photography stand (I had a few years in Kodak's Image Simulation Laboratory in Rochester, NY), I'll take better glass lenses over a better camera almost every time. Larger lens, and better lens glass will almost always produce better image quality potential. The GoPro lens is so much more substantial then a cell phone's lens, I can't help but believe they are far superior. I just purchased a HERO 7 Black for a cruise, and used it for both above ground, and SCUBA imaging, and I have never seen anything off a phone come close to the image quality, and my wife has the new iPhone XS. Last thing to consider is viewing condition. If you'll never watch the video on anything other than a cell phone screen, it won't matter as much. If you'll be watching it on a hi-res monitor or large-screen TV, you'll see the difference for sure. Just my opinion, but I would always chose the GoPro over a cell phone for imaging if image quality was important.
