In theory it really shouldn't matter. You choose your instance, and it's up to the instance admins to make decisions about backend software choices. It's possible that we'll get to a place there it's possible for admins to migrate a server from Lemmy to Piefed or back again without loss of content, in which case all the user would see about it would be a change of default interface.
I'm on Feddit.uk, which has several different web interfaces to choose from, and I mostly browse using a mobile app (Boost). It really makes basically no difference to me whether it's running Lemmy or Piefed.