just saw this, dont know if it was too late or not, but here's my 2 cents:
both are greats, have made classic music, but i voted pac for 2 reasons:
1. pac did something nas didnt until later in his career: he perfected being able to touch you no matter where you were from cause of his use of common language in his raps...no matter where they were from, they could relate to his music...
2. pac sounded more comfortable rapping about any and everything, while nas doesnt....when nas does club or 'for the females' type tracks, he doesnt sound comfortable....where as pac sounded comfortable no matter the subject...it just felt more natural....