I agree with Emerald. Mainstream rap < Underground rap
Here's the definition of the 2 and how they are different by my understandings.
Mainstream - Degrading women, don't speak facts when they're rapping, and lacking the passion. They care more about money, fame, and fashion than they're lyrics. It's been proven, MAINSTREAM RUINS ARTISTS. Look at Wiz Khalifa, one of the biggest sell outs in hip-hop today. His Show and Prove album was "facts", had his own style, etc. As soon as he went mainstream he turned to utter crap. He only raps about drugs, money, women, and fashion. Underground he rapped about the streets, how he had real flow, pretty much for the passion than idiotic ignorant crap.
Underground - As a whole speaks truth. When they're rapping they're speaking facts and have true meaning. They don't rap about crap that degrades women and drugs, money, etc. But just because your underground don't mean your real, I've heard a few underground artists that are fake as mainstream. That's why as a whole they do it for the passion. Lots of underground artists are smart enough to not get sucked into the mainstream and have made MANY songs about mainstream or mainstream industry music.
I'm gonna name a few "underground" rappers that will completely murder anyone on the mic in mainstream like they were goin on killin sprees. Tech N9ne, sickest flow today and even tho he's a chopper at times (spittin super fast) he's a lyrical genius. He's rapped about the industry on numerous occasions and everyone in mainstream know's he's the king, 40 years old and still has the stamina he had when he was 25. He's a legend in rap. Songs like Come Gangsta, Industry is Punks, Who Do I Catch, and Ugly Duckling are a few where he bashes the industries. He also raps about struggles and the half owner of Strange Music. The dude makes millions upon millions and yet made songs like he went mainstream. He respects mainstream but hates them at the same time.
Hopsin, yet another underground artist and the owner of Funk Volume. On numerous occasions rapped about the industry. Nocturnal Rainbows, Ill Mind of Hopsin 5, Kill Her, etc. Hopsin is yet another lyrical genius and went from homeless to rich. The mainstream despises his music and they told him he's not "real enough". He pretty much said fy I'm gonna make my own label, and look where he's at today. As soon as he started getting recognized mainstream industries have been blowing him up with messages and his latest song "Independent Living" mentions that. He's making close to as much as mainstream artists and has some of the realest lyrics known to rap.
Immortal Technique, he's been down the mainstream industries road and he knows what goes on. He's one of the realest rappers in the underground and owns his own label as well.
Mainstream has no reason to do what they do because they're cash flowin, it's actually not them, it's the industries and they're just puppets.
As Tech N9ne said "Yeah the radio dead! - cuz the industry is punks! (punks!)
Put the double-barrel to it's forehead!"