Only thing about Big Boi is that he has some Silk The Shocker moments on his past albums. His recent shit is on point, though. But like on that song "Tough Guys", Big can fall off the track trying to rap some shit.
The only songs where Bun almost lost it was "Country Shit" with Big Krit and on the song "Corrupters Execution", but he catches it back rather quickly. Outside of that, his flow is ... well... impeccable.
I think Big has Bun lyrically by a few hairs, though. Bun doesn't kick the metaphors and other extra shit as much a Big Boi does. Then again, some people don't care to hear all that in a song. They just like it to the point.
They're both good, so you're gonna have to knit pick with these two.