I think if I had my choice, I would have picked Brees. But, the way free agency worked, I liked being the only team able to sign Warner before March 3rd. It would have been a huge risk to go into free agency with no QB and hope to win the Brees sweepstakes. If you lost and Warner signs with another team, the Cards season would have been totally screwed. Also, Warner is cheaper this way.
Also, with Warner signed, it looks to the rest of the league that the Cards have their act together, so the early free agents would be more likely to sign here. Otherwise, many free agents would wait until the Cards QB position was determined before signing here.