I've only been to one Six Flags Park, Six Flags Over Texas, and as someone said earlier both chains have their strengths and weaknesses. I'll explain briefly...
Going into my visit back in 2007, I was expecting to enter a very bland and generic corporate park with lots of rides but little to no charm, as a lot of what I had heard about Six Flags parks was similar to what has been posted by some in this thread. I was very surprised with what I found. The park itself was beautiful and I would have to say it was more of a theme park than Kings Island is, as for the most part the rides all fit their respective areas, even if I didn't care for some of the themes (ex. I prefer Congo Falls "theming" over Aquaman for a splash ride and I think Zephyr is a much better looking swing ride than The Gun Slinger, etc.). The rides were good and I had a great time, of course I was also dropped off and didn't have to pay for parking. My day at Six Flags Over Texas left me wanting to get to more Six Flags parks, something I've yet to do, but hope to change soon.
Six Flags and Cedar Fair are both good park chains, do I wish Six Flags owned Kings Island? Not necessarily, but I don't think it would be the end of the world and they would probably do a lot of things I'd like, as well as some that I wouldn't (advertising wraps on coaster trains anyone?). I was one of the people who, after hearing CBS wanted to sell the Paramount Parks, was keeping their fingers crossed that the parks weren't made into a package deal and that Kings Island would've become a Busch Park (now SeaWorld Parks). When I heard Cedar Fair purchased the parks I was a little disappointed, but they have surprised me and done a lot of great things for the park, as well as some questionable things.
I've gotten a little sidetracked but my point is that you shouldn't overgeneralize about park chains, they all have their strengths and shortcomings, their strong parks and their less than strong parks. The end.