Six Flags definitely has more thrill rides. Depending on where you're staying, the drive there could be 1-2 hrs. If you're staying near DL then Knott's is within 1/2 hr. Knott's is fun too but doesn't have as many thrill rides as Six Flags. Also it depends on what time of year you go. Six Flags can get very hot! Both parks do get crowded during the summer, especially weekends. Both parks have a separate water park located nearby.
As far as safety concerns, I wouldn't worry. Both Knott's and Six Flags are run by large companies and I don't think they'll take any unneccesary risks.