I don't know. They both won a Super Bowl, Favre won three MVPs, Rodgers won two. I'm sure that the QB stats favor Rodgers, but he wasn't able to win big games in the same way that Favre could do.
I compare the two this way. Both my mother and my grandmother were great cooks and bakers. My mom studied chemistry and understands the science of cooking and baking. My grandmother was of the ilk where you take a handful of this and a pinch of that. Rodgers is more the scientist working on perfection while Favre was more the winging-it QB. Different styles.
Luckily this thread is about who we would each rather have, rather than trying to determine who would be the best.
Both Favre and Rodgers were exactly 12-10 in the playoffs. Favre had a winning percentage of 63% as a packer; Rodgers was at 66%. They both won 1 Super Bowl. So I don't think there's much basis for arguing that one was better than the other at winning big games.
A lot of people dismiss the statistical superiority of Rodgers based on era, but that doesn't hold water for me. Rodgers broke a passer rating of 100 twice while Favre was still in the league. They weren't playing in dramatically different eras. The actual reason for statistical superiority is that Rodgers was much better when it came to throwing touchdowns and protecting the football (i.e. the stats are illustrating an on-field superiority).
Rodgers: 6.2 TD %; 1.4 INT %
Favre: 5.0 TD %; 3.3 INT %
Throwing touchdowns and not throwing interceptions are far and away the two biggest ways that quarterbacks can contribute to winning football. And Rodgers smokes Favre in both.
And then you have dramatic difference in ancillary stuff that just widens the gap.
Rodgers: ~3500 yards rushing; 35 rushing touchdowns; 92 career fumbles
Favre: ~1786 yards rushing; 13 rushing touchdowns; 166 career fumbles
This is why I say there just really isn't any contest. Rodgers was clearly the superior player. Hence, why the count-arguments tend to be about Rodgers as a person (berating receivers, not trusting young players, etc.) or about stylistic preference (e.g. someone just loved Favre's gunslinger mentality).
But the bottom line is this: Favre is easily a Hall of Famer and in conversation for one of the 10 best QB's of all time; Rodgers is in the conversation for the single best QB of all time. That's a big difference.