Between the two, what should I get? My threat model is to avoid advertising & related tracking, and local files should not be snooped on.
I’ve also found that nextdns has a samsung specific native tracking protection filter
Between those two options, I’d go with the iPhone.
But @anon96490812 is right that you have more options for youtube frontends with Android (and open source apps more generally). There are options on iOS though (Yattee, Piped as a PWA, Safari + Adguard, or Brave Browser) but none are as polished an experience as Newpipe on Android.
I switched to my first iPhone when the iPhone 13 came out mainly for no preinstalled google apps or facebook and other apps the phone brand get paid to preinstall. Yes you get more apps on fdroid but if you do not want to wipe your phone and add coustom rom pick the iPhone. I just watch youtube videos in brave on IOS! You can also save most videos on the web in brave playlist for offline watching for free without downloading the video and i find that better than a MP4 file from newpipe on android. Also less data collection on iphone, Samsung phones have both Google and Samsung apps and appstores collecting data.