If Brett Favre is a Viking in 2009, the only way it can be considered a success is if the Vikings win the Super Bowl. And that makes the risk far outweigh the reward.
Partly, it's practical. I doubt very much Favre has more than one season left in him, and that means signing Favre is a one-year fix. This means if they don't win it all with him in 2009, then all they did was set the franchise's long-term quarterback search back one season. If Sage Rosenfels is successful, he could be the team's QB for the next five seasons.* No matter what Favre would do, he would only be the quarterback for one season. So if they sign him, they push back finding out who should be the team's QB for the next five seasons.
Partly, it's emotional. All I really want as a sports fan is to see the Vikings win a Super Bowl, and if it takes Favre to win it, so be it. But if they don't win it, they'll have tainted far too much. We'll have a season of actually rooting for the nemesis, and for what? For a Wild Card? For a division title? For a trip to the NFC Championship game? It's not worth it.
I think Sage Rosenfels might be a five-year QB. I'd like to find out if he is. I want to see the Vikings win the Super Bowl, and I think finding a five-year QB gives them a better chance than signing Brett Favre--who has not exactly been a reliable playoff QB--for one season.
*Yes, I consider a five-year QB a long-term solution. Obviously you want to find a decade-long QB solution, but that's rare, and there have been a lot of successful QBs--even at the Super Bowl level--with shorter runs on a particular franchise (Rich Gannon, Kurt Warner, Brad Johnson).