i don't think that this is fair comparison, because nash was a pure point with shooting guard skills, and james is a combo guard at 2/1. nash's catch and shoot was better, ball handling skills were better. james can use his strength and drive in the lane better for sure.
I think Harden's the more talented player. But Nash was the better basketball player & made others better at a higher rate than Harden did. I'd also trust Nash with the ball more in clutch moments than James Harden. Nash ofcourse never quit on his team too