I'm telling you....it's mainly the voice and delivery.
That's it. That's what really separates the 2. Non-Southern rap fans find Andre's rapping more accessible than Antwan's on average.
People say it's because Andre has more substance but Big Boi has plenty of songs both in and out of Outkast that are introspective and personal. He just balances it out with the player and flashy talk. They both have moments of them snapping on verses and Big Boi is actually better at double time flow than Andre by a mile.
The real reason Andre is seen as much better is because his delivery and flow is much smoother and easier to digest than Big Boi's. Big Boi is from Savannah and still has a thick accent and drawl. Andre's from Atlanta and has a more neutral sounding accent. Andre's flow and style was also more influential to rappers like Kendrick, JID, EarthGang, etc. compared to Big Boi who has a much more limited influence.
Also, Andre has more of a unique persona due to his fashion choices and he stood out as the "quirky" one in Outkast when he started singing and experimenting with genres on Stankonia and The Love Below. Then, Andre cemented this further with the Class of 3000 soundtrack and now the flute album.
Big Boi also experiments with R&B, Funk, Indie Rock, and even Techno but he's much more sublime with it and Big Boi's whole persona is being a pimp/player unlike Andre which a lot of hipster types resent as an image. Hence, they're less likely to give him props and more likely to label him as a "stereotypical" rapper who only raps about bytches and other vain stuff.