For regular users there is not a huge difference, but the web-interface is significantly faster and you can subscribe to topics that combine multiple communities. The disadvantage is less mobile app support. Right now only Interstellar supports Piefed.

The real advantage is for moderators and admins, as the Piefed developers actually listened to community feedback and implemented a lot of nice moderation features that require bots or annoying work-arounds on Lemmy.

And there are a few technical differences that make Piefed easier to administrate and troubleshoot from a sysadmin perspective.