Plex, to it's credit, does make streaming externally from the home network easier. Setting that up with Jellyfin is a little more involved, but it's also free, whereas Plex will make you pay for that. But if you have no desire to stream outside the home, it's not an issue.

Jellyfin apps on other platforms are a bit of crab shoot. Some are maintained very well, some (like the Android TV version) have fewer mainteners and go a long time without updates or fixes. For most users, they're perfectly adequate, but it's something to be aware of.

Plex's app support on various platforms is better, but much less controllable and customizable. That goes for the main UI as well. It's polished but you're stuck with whatever Plex decides to put there. You can customize Jellyfin much more, strip out things you don't want, etc. You can apply custom CSS, too.

Plex is a business, and therefore it has things it wants you to see whether you like it or not. The enshitification of its UI will get worse overtime, as happens to all for-profit tech company products, but for the time being it's tolerable. Just don't get too comfy.

Overall I'd suggest Jellyfin for most in-home use cases, and if you're comfortable managing external connections (and the security of it). If don't have the time or knowledge to manage this beyond powering it on, open the wallet and go with Plex. But there's no reason to pay a subscription for something your home equipment and your Internet connection are all doing on their own if you can spare a little time to set it up.