I’m late to this question, but I really like it, so I am crashing.
In ArM5 authors and editors have worked very hard to give each Tribunal distinctive character so that, in many senses, there’s many different official ways to play Ars Magica. I admire this very much, even if some of those official ways don’t interest me much. But I have run sagas in many tribunals, and here’s my two cents:
Rhine: This is all about guild politics and the gerontocracy of older magi stifling the young. Of course there’s wonderful local folklore to interact with, but that’s true of every tribunal. It’s the (usually) non-violent intrigue and Hermetic politics that distinguished the Rhine.
Theban: This is the place for PCs who want to cooperate with one another and the Order at large. Conflict between covenants and magi is discouraged, but sagas need conflict, so the PCs spend most of their time pursuing their own shared interests or combatting non-magi antagonists like the Venetians or monsters from Greek mythology.
Normandy: One of two ArM5 books obsessed with violent but usually non-lethal conflict between covenants and magi, Normandy is distinguished by its emphasis on feudal culture. So when you fight other magi, it’s usually in a tournament, or you’re on a raid and are limited to certamen.
Hibernia: The other tribunal obsessed with violent but usually non-lethal conflict between covenants and magi. Hibernia is distinguished a Peripheral Code with violent customs (Macgnimartha, Wizard’s War never expires, defending your covenant against repeated assault by other magi). There’s a reform effort that places Hibernia in tension with the rest of the Order, but it’s futile unless the PCs want to join it (but if you want to make Hibernia more like the rest of the order, why play in Hibernia in the first place?).
Transylvania: Yes, it’s all about Tremere, but more importantly, it’s ArM5 Tremere with their “the right thing for the wrong reasons” theme. This tribunal is distinguished by the fact that it’s incredibly well organized and magi can operate in the open, so it’s as close to a magi-run nation state as you can get without the PCs founding their own such state. Horror themes are also strong enough that they get a whole chapter of the book.
Provence: This is specifically designed to be welcoming to new players. It’s the least different from the core rulebook, it’s easy to form a new covenant, and there are large powerful covenants the PCs can join by simply visiting and never leaving. It’s got supervillain-level threats and a cool dungeon to explore. If your players are new to Ars, it’s perfect.
All of the other tribunals are from before ArM5, and that brings a lot of problems. Stonehenge is supposed to be focused on House Tremere as the villains, but the revision of Tremere in ArM5 means Blackthorne no longer makes sense as written. However, there’s one great reason to use Stonehenge anyway, rewriting it as necessary: all the research material is in English. If English is your native language, and the first language of most of your players, being able to read primary sources and most of the research materials about the region where your game is set is a tremendous advantage. Research has come a long way since the 1980s when this game was created, but language is still a barrier, and if you like doing it, and English is your first language, Stonehenge is very, very compelling.