I'd say HHH/Brock was easily the bigger sell for the PPV than Taker/Punk. Look at which build was more serious (hint: the one that involved Vince) and which build was half-assed. Taker's matches will always draw based on the streak, regardless of who the opponent is. Let's not forget the numbers they did for Summerslam, which is one of the main reasons why the match happened again in the first place.
I agree that Brock/Bryan could make Bryan look like more of a legitimate star but that's why I'd do it on a lesser PPV than Wrestlemania. I also wouldn't put Bryan over in a million years. I'd book it as Bryan putting up a good fight, maybe even locking a submission hold or two on Lesnar and getting him to scream in pain (would be cool to see Bryan break out some submissions we haven't seen from him yet) and keep on fighting despite the punishment he takes but in the end, he still falls to Lesnar. He'd be put over well enough just by hanging with someone like Lesnar that long. It is a dangerous (though if he wants to be taken more seriously, necessary) thing for him to get away from the comedy bits of his personality though, because it's the reason he's over in the first place.