I'd say the Cardinals are the bigger surprise. I'm pretty sure nobody except for Cardinals fans and players saw them going into New England and getting a win there. This is the same team that had no clue who was going to be their starting quarterback, and once they did he got injured right away.
While I'm surprised that the Saints haven't won a game yet, I knew they would struggle not having their head coach and coordinator to start out the season. I think people downplay just how important their presence is, and how much pressure it has put on the team in the meantime,
While both are surprising, I have to go with the Cards on this one.