Kayako spirit is alot stronger since she can infect anyone who has contact to the house or the person who entered the house.
Sadako is harmless unless you watch the tape and even then, you got 6 days to live. Kayako kills within a few days upon contact. Not to mention Kayako killed so much more than Sadako
But you can copy the video, give it to someone and avoid death, once you entered Kayako's house there's no way you can live, and that hasn't changed in any movie.
I would imagine Sadako would cause much more damage since it's much easier to come across the video (since everyone makes copies to stave of being killed/mutated into Sadako) than to visit the house. You also need to remember that Sadako's curse isn't to actually kill but to clone herself. She uses a mutated strain of the smallpox virus to spread after all (actual backstory from the book). Many people die in the process yes but that's not the original intent.
Disregarding Loop in this since that made the story even way more insane than it already is.
Did you ever notice that people who believe in creationism look really un-evolved? - Bill Hicks
Kayako's power seems more deliberate. She is like a fast striking spider, once you are in her web you are as good as dead before you can even react.
Sadako on the other hand is like a jungle cat, a cool and collected predator who toys with her prey.
In a combat comparison, Sadako seems to have the upper hand in intelligence. She gives signs of being very sentient, while Kayako is more like an engine that runs, or a creature of instinct following a pattern.
To me, Sadako intends her curse to be like that, she could kill everyone instantly but she decides to play with her prey, give them a few days to think about what happened to her, even gives them a way out of they spread her suffering so the whole world knows what happens.
I'm the guy who makes the "worst movie ever" thread in your favorite movie board
Kayako is more of the physical combat type of ghost. She's way more violent and bloodier than Sadako but has less psychic/supernatural abilties than her.