there isn't really a benefit to using both, just pick whichever one you prefer and use that

https://join.piefed.social/features/

It's newer and new features materialize faster. Also the dev(s) are not tankies. But they also added an (optional) feature where upvotes are limited due to excessive influence of most active users? IMO this space is still too small for that to matter. If I want to spread positivity and reinforcement on contributors why should I be capped?

Is PieFed superior to Lemmy?

no similar, some difference in the minutiae. Mostly its about Lemmy Devs not being liked by some folks, hence piefed.

I have a piefed account, same name, as a backup when The Lemmy server I use shits the bed (once a year or so)