Heres a completely different perspective. Just got back from my first ever experience of both as I was on holiday (vacation) from the UK.
First park was SFMM. To keep it brief I got lucky from what Ive been told so far. The rides were nearly all fantastic and there were loads of them. Every single one of them was open! Plenty of coaster variation with so many different styles. Schools had gone back and there were barely enough people to form a gang so I cant comment too much on clientele. Food was quite good. Theming was atrotious. Gotham City was OK. Staff couldnt give a hoot. All looked bored and disinterested. Only one show and it as closed.
Next, Knotts Berry Farm. Fantastic park. Theming was much better, not great but certainly good. The shows were great especially the Wild West Stunt Show and the Saloon show as I ended up taking part!! Rides were a good mix. XLerator and Silver Bullet and Supreme Scream for the extremists, Boomerang and Montezumas Revenge for the slighly less brave, Sierra Sidewinder for brilliant family fun, Roaring Rapids to get damp and Riptide to get drenched through to your underwear!! Ghost Rider was closed but I think its opening for Halloween Haunt. Cant recommend Sierra Sidewinder enough went on 7 times! Food was OK and shops were quite well priced. Camp Snoopy Land was good for kids and big kids.
So to summarise I only have the benefit of 1 park from each provider. Based on that I would say if your with a family, KBF representing CF offers a great all round family day out. MM representing Six Flags offers a fantastic day of high adrenaline extreme coasters for the teenagers and wannabee teenagers!! If I had to call it I would go with Cedar Fair