The only difference I see is the better ISO performance. I will not spend 3000 dollars to buy 5d3 unless I want ISO booster.
Ah... but it's the differences you DON'T SEE that really make the camera worth the upgrade [for my shooting style]. I wouldn't have upgraded on an ISO performance boost alone. But the AF system alone... that's a different story. Also, you'd be paying 3500, not 3000.