Hi Winston112,
We live in Ann Arbor so, not so far from you....
We've been to all three parks you named and I would say it all depends on what you can and cannot do. (Surgery wise I mean.) Six Flags (I will forever call it SEA WORLD!!!) would be a good choice if rides are a problem for you. Lots of cute show. My girls enjoyed all the sea life.
Can't remember a lot about the park in ON. Didn't make a great impression on us I guess.
Our very favorite park is Cedar Point! We've been 3 times since May and are planning another trip next week! Yes, we are making use of our season passes!
You say your son is 6 and I think that's a great age for this park! It's know for all it's huge scary rollercoasters, I know, but we never ride them and still have a great time. I have daughters, 4 and 8 years old. They love Camp Snoopy. Both are big enough to ride the midway rides and now the Snoopy ice show is open!
Soak City is suppose to be very fun!
Cost wise I'm not sure how it compares to the other parks. I know there are places (like Meijer) to get discount tickets. We stay at Hotel Breakers when we go over night. Very nice, but there are lots of other hotels all over!
Have you checked out their web site?
There is also Great Bear Water Park up the rode from them. THey have a web site too.
Let me know if I can answer any other questions.