Both are very good rappers, in my top 10. Both have consistency, but a different kind of consistency.
Jay Z had consistency of celebrity status, but because he was willing to constantly change and adapt to whatever standards society has. No other rapper has been in the limelight for 16 years in a row.
Nas had consistency of his rap technique. He is just as lyrically talented now as he was in Illimatic era, and his rap style is still very similar. No other rapper has sustained that amount of purity.
Jay Z ended up selling better because he was able to change whereas Nas didn't modernize as much.
I'm gonna have to say right now it's a tie. Because I can't decide. I love both.