Having taken many trips to both, here's an honest comparison:
Cedar Point: pros
-Fantastic coastering in anyone's opinion
-Its surrounded by a lake, good view from up high
-Very clean, capacity second to none
-Lots of places to rest, things seem together in the park.
-Lots of family things to do
-Great operations
-Really quite the memorable experience every visit
CP: cons
-Other than Disney, the undisputed KING of long lines and crowds
-Pretty much no theming efforts, other than the minimal Disaster Transport.
-The woodies are far from enjoyable.
-Visitors expecting too much, stemmed from overflowing attention and publicity
-A couple of coasters are painful
SFMM: pros
-Easily one of the more beautiful settings ive seen theme parks
-In my opinion, all of the coasters are enjoyable, especially when they're open
-The BEST views from up high
-Park circulates very well, easy to go were the majority isnt!
-Did I mention the beautiful terrain that the coasters take advantage of?
-Lines rarely exceed 1½ hours
-Bad reputation (?!?) makes for a wonderful surprise
-Queue lines are very bearable, with good music (my opinion again) and above-par theming. Lot's of darkness and colored lighting!
SFMM: cons
-some coasters are tamed from original ferocity, and have that feel of "that could've been fantastic!"
-Food services is pretty lowly, need a beefier staff in this feild.
-Woodies can be moody
-Flashback rarely runs, and you cant ride single! (grr...)
-Teens can be dumb, as the always are (not nearly as bad as SFGAdv was in April)
All opinions of mine, but I figure they're fairly general and can give you a good idea why certain people like either park more or less. For my vote, I like Knott's Berry Farm best!!