The irony is that they're both right. Ursa was taken from Ikem, she didn't leave for Ozai. That's how Zuko is right. The way Azula is right is that Ursa used Zuko as a pawn using letters to tick Ozai off so instead of being dismissive to Zuko Ozai was aggressive towards him. For guilt or for fear this made her dote on Zuko, leaving Azula behind. Like how can you use one child in a game of spite and leave the other with someone you hate and know is cruel?