I paid for Plex and have never seen any ads. I tried Jellyfin and required too much tinkering. If you spend more than an hour tinkering with Jellyfin it's worth it to pay for Plex lifetime and be done with it. A good example of this is using it say on an Apple TV or more niche devices.
That being said, I do like Jellyfin and it's fine for tinkerers. Things like sharing, watching together, etc are a huge pain with Jellyfin.