They are too different to break it down and say one is better then the other, all comes from opinion. But seeing thats what you asked for I'll give it a shot.
Joe Buddens has the ability to rhyme songs with alot of variety, from being meaningful and deep to coming of slightly introspective and collective. His concepts are dope and he as an emcee can go hard with punchlines and disses in general.
His ability to make an album is solid, but there's only 1 to his name. He can cater to different people's likes and even when trying to come off commercial he sounds half-decent.
As far as technicalities go, his flow is around mediocore and rarely really comes through with that "let me rewind that, that flow killed it" feeling. He can however eg. 'Focus'. He had a sick flow and delivery on that song. His lyrical is descent, I see alot of Biggie-G Rap and Kane incorporations inside. His mixtape game has been exceptional!. He has murdered a few mixtapes this year.
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Ras Kass is one who is considered an elite, well rounded emcee. Diverse and lyrically one of the best by many. Do I consider this? Yes, slightly. At first he had a real mediocore flow however his rhymes and concepts were extrodanairy. eg. 'nature of the threat' & 'interview with a vampire'. Has be progressed he got better with his flow but I think his lyrical slightly stepped a few steps down.
His got the same ability to make deep, meaningful songs. He has some real nice punchlines, Ive never heard him really go hard against anyone. He did get at the Alchemist but it was the usual "yeah he a bitch" type of shit. He to can come introspective and collective.
He probably does have crossover appeal but we haven't seen it yet. He is at best when working with people like Killah Priest & GZA or Xzibit & Saaphir rather then Dr. Dre & Kurupt. I rate him one of the wests best, "Institutionalized" was a solid effort. He can make great records too with catering for different fans "Van Gogh" & "Goldyn Chyld"..
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Id say Joey Jump-Off is a 7.5/8 from 10 emcee at this moment, while Rassy Kassy is a 8/8.5 from 10.
Im going with Ras but Id rather listen to Mood Muzik 2 over Instituyionalized and Joe's debut over Rasassination. Ras Soul On Ice is on par with classic material though.
peace.