They are both the same size. They are both lefty. They both possess insane passing ability, and the ability to get into the lane when they need it. They both are good but not great athletes, and they both have good, but not great jump shots.
Harden has a tighter handle and a slightly better jumper, but DJ is a better rebounder. Harden is a little smoother with the ball, and has a very nice crossover and stutter-step, but he was also a top 3 pick.