Your claim was proven last time Milan faced United
At the time Kaka was better, that time has passed and Ronaldo has been better for two years now
Ronaldo is faster, better dribbler and superior in air.
Kaka is better at everything else.
Like Hoori said Ronaldo is also better in scoring goals and has got a more powerful shot on him. I'd also add that he's stronger physically, plays better with both feet, his off the ball movement is also immense, he's more versatile, he's better at taking freekicks and has higher fitness levels than Kaka, not to mention he has proven to be more consistent.
So thats, faster, better in the air, better goalscorer, stronger shots, better freekicks, more versatile, better off the ball movement, higher fitness levels, stronger physically, more versatile and more consistent.
Kaka is a better dribbler imo, he's got better vision and he's a better passer.