Since the decision pertains more to the players than to the characters I'll bring this here.
For Regulus both Rhine and Normandy are fine. He is slightly inclined towards Normandy because he spent half of his apprenticeship there, but overall both present opportunities and challenges to his goals.
From a legal standpoint, Normandy should also be easier to join (because we already meet all of their requirements to establish a covenant), but we should expect more trouble to defend our vis sources in the long run.
Talking as Rafael: I like Normandy a bit more than Rhine, seeing it as a more dynamic Tribunal. I lean slightly towards Normandy just to spice things up, but my view is that in the long run we'd have more politics and a bit less vis if we go that way. Chosing Rhine we'd have less politics (but still some, because Gilds). Chosing the Alps, after establishing the covenant, we might get away with no politics.
Finally, canonically Regulus went to Confluensis to peruse the Tribunal records of Normandy. My reasoning for that at the time the decision was made:
He would be departing soon after the events in La Roche D'Or (or returning right before them, depending on when they occur). It would be an opportunity to get one of the magas traveling with him (I could run a short story if there's interest), or to find a French companion.