Personally, I not only prefer Nas, but I think he is the better rapper when going at 100% effort. Obviously since Nas was hungry in the beginning, Illmatic is an album that he will never surpass. After that, most of his albums were mediocre, but then Jay-Z got things heated, then Nas almost made a total comeback with Stillmatic. It was almost better than Illmatic, and "Ether" is an unfuckwithable diss track that demolished anything that Jay-Z said before or after. As a rapper, Nas has an incredible lyrical arsenal full of crazy wordplay, similes, metaphors, imagery, and creativity. But Nas's problem is that he is inconsistent and really needs motiviation to drop classic albums. Jay-Z exploited those weaknesses in "The Takeover" and got Nas gassed up.
Jay-Z has a lot of rhyming skills and utilizes multi-syllabic rhymes more often, but his lyrical content gets redundant and often gets overused. He's usually talking about hoes or something, and these songs get a lot of club play which I can't hate on, but he doesn't explore boundaries as an artist that often. But when he does, it's awesome... just listen to songs like "Song Cry" or "Soon You'll Understand" (definitely one of my favorite Jay-Z tracks) or "The City Is Mine". Now that I think about it, when considering subject matter, Jay-Z to me is like a Cash Money artist that can actually rap. But Jay-Z knows how to make albums that will go multi-platinum and still get praise from hip-hop magazines, plus he's a businessman, he's got his money right with the record company thing, Rocawear, plus I heard that he was trying to buy partial ownership of the Knicks before.
So yeah... Nas.