I went to Magic Mountain this February, but haven't been to Cedar Point in 3 years.
My advice is go to both. They are both unique and fun.
Magic mountain has X2, Tatsu and a terrain to make a rather slow coaster like Ninja a fun ride.
Cedar Point has Millennium Force, TTD and a great location on the lake.
I'll be at CP in July to refresh my memory and ride Maverick for the first time.