If Jellyfin covers all your needs and you're already familiar with it, there's not much reason to change to Plex.

Plex pass will let you skip intros and credits. It's a nice-to-have. It will let you download your media to your phone/laptop so you can view it without an internet connection. Also a nice-to-have, but not a huge selling point.

I like Plex for how easy it is to access my media from outside my network. The apps are great, and it's easy to add family and friends so they get access too. With Plex pass you can give them filtered access - great with kids in the house.

My Plex server runs from my Synology 920+, and I've never had any luck getting transcoding to be a useful feature - lots of freezes and buffering. Better to have the arr's grab media in a format that will direct play on the client and not bother with the headache.

I use PlexAmp and it's a very good music player on par with anything else.

Nothing is so good/problem free that I wouldn't replace it in a heartbeat the moment Plex does some questionable changes to the service. I got my Plex pass many years ago at a third of the cost that it is today, and I didn't really know what I was getting, other than making the Plex app free for everyone logging in to my server.