From a gameplay perspective Berseria is essentially identical to Zestiria. The differences, such as now being able to learn item skills (which partly addresses the earlier issue of items not being straight upgrades of one another), are fairly minor.
The differences are more pronounced when it comes to storyline and environments.
Here you play as an anti-hero who’s often fairly ruthless as opposed to a more altruistic savior-type, which goes a long way toward making the events feel fresh. Hampering this are exterior locations that feel especially claustrophobic/cramped compared to Zesteria. It’s as though they took the design strategy from that game’s (boring, grindy) dungeons and applied it to the whole game, resulting in an extraordinarily unpleasant experience. It feels like a series of corridors.
Maybe it will open up later (I’ve only just unlocked a full 4-person party). If it doesn’t, then theme and minor technical improvements aside, there won’t be any reason to play it over Zesteria.