Spitting can be way more technical, lyrically. You don't hear much rock or metal with multi-syllabic rhymes and flow like some hip-hop has. But it's honestly hard to compare the two. They each require a different type of skill. I can't say which takes more practice as I'm not an emcee or singer for a metal band.
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Thanks for the good answer. I agree that both are difficult and require different skills to master. I personally don't think I could be very proficient as either a metal vocalist or rapper.