I've been using both for ages.
For remote access to friends plex is easier and cleaner.
For offline viewing in Android plex is cleaner
I'm running tailscale with jellyfin for personal use and it's wonderful, But I wouldn't ask my relatives to do that and I don't trust to surface the port. Plex has a dedicated security team and 2FA.
The Roku client for jellyfin is also a futureless husk of a client.
I have lifetime Plex so I'm in no hurry to do a full conversion. I would love to drop plex all together though