I believe in Bryan over Reigns because of one simple fact. The fans are being led, not forced, into cheering for Reigns. On the other hand, people have gone out of their way to cheer and support for Bryan. They got him to the main event of Wrestlemania. Those little boos that Reigns has started encountering when facing opponents with loyal fans like Rollins could potentially lead to bigger boos.
That's a big factor. While a lot of fans do genuinely like Reigns, like me, he's nowhere near as over as Bryan has been. Whether or not Vince ever intentionally decided to push him as big as he was is incidental, Bryan's popularity with fans was something that couldn't be ignored. Whether it be casual fans, average fans or hardcore fans of men, women or kids, people
WANT to cheer for and
LIKE cheering for Daniel Bryan. If Vince ultimately wants Reigns as the next John Cena, that doesn't automatically mean it'll happen. Because Reigns fits the "type" of guy that Vince prefers to be
THE guy, namely this big ol', muscular stud, some fans will turn on Reigns just because he's that type and because Vince likes that type.
Besides that, having the look won't be nearly enough in this day & age especially. If fans don't want or accept Reigns in that role, it won't happen because they won't invest time in him. Or, I should say, they won't invest the time and interest in him that would be required for the next "face" of the company. Cena's also tirelessly devoted himself to WWE with a work ethic and sense of loyalty that can't be questioned by even the most vehement Cena hater. Plus, fans also want to rally behind guys who they can not only connect with, but who can put on classic matches with great storytelling. Compared with guys like Cena, Bryan, Punk, Ambrose, Rollins, Cesaro, Wyatt, Ziggler, Barrett and a few others, Reigns just isn't in the same league right now whether it be inside the ring, on the mic, as a character or some combination of any or all of the above.
as for Bryan vs. Lesnar, it's a match that pretty much writes itself. Lesnar is one of the few guys in WWE right now that can genuinely put Bryan in the role of underdog. Lesnar's been dominant over the past year or so. Probably CM Punk's last truly great, classic match in WWE was against Lesnar at SummerSlam 2013, Lesnar decimated Big Show & Mark Henry early this year, he broke The Undertaker's 21-0 WrestleMania undefeated streak and he gave John Cena the most dominant and decisive loss of Cena's entire career. It'd be David vs. Goliath and it's a role in which both men can shine. Heyman as Lesnar's mouthpiece has given, arguably, the best promos of the year and Bryan can deliver intense & passionate work on the mic, especially with someone that he can bounce off of like Heyman. I think people would rally behind Bryan and would want to see this "normal guy" rise to the occasion. Whenever WWE's given Bryan a genuine opportunity, he's taken it and made the most of it whether it be his push/heel turn as World Heavyweight Champion in 2011-2012, his feud with CM Punk during the summer of 2012, forming Team Hell No with Kane or his push as a singles main eventer beginning last summer.
If Bryan is as popular as he was, and I see no reason to believe he wouldn't be as there's no one that's come along that's gotten anywhere close to his level of popularity since he's been out, I see more money in Bryan vs. Lesnar than in Reigns vs. Lesnar.