There's many ways to look at it, but I think most people would want the 49ers to win this week. If the 49ers do win, the Saints fall to 7-3 and will have the same record with the Niners and we'll be in better position to keep the #1 seed in the NFC since the Saints are the closest team to our record and have just lost. The Niners will be the #5 WC. Also if this scenario happens and CAR or DET win, they would need a tiebreaker to determine the #2 seed since they're all 7-3.
If the Saints win, the Niners fall to 6-4. The Saints are 8-2 and are still not far behind us in the NFC. That being said, if ARZ wins, they'll also be 6-4 and tied with SF in the division and their places would need to be determined by tiebreakers. If SF continues to lose and AZ pulls off some wins, we might see SF out of the playoffs completely and AZ in their place instead, but it's a little far fetched.