I agree Bryn looks more Welsh/authentic. But to me, Bryn is very masculine, so I would cringe even more to see that on a girl. Brynn looks more feminine, so if it must be a girl's name, I'd prefer to see that spelling.
Actually, an acquaintance of mine recently had a girl and named her Brinn and I surprisingly really like that. Then it doesn't even look like it's fighting to be the same name, and it seems much "cleaner."
This message was edited 5/10/2015, 2:53 PM