Circumstantial evidence includes EVERY type of DNA evidence. The only other type of evidence in a court of law is direct evidence. This might be a confession, a video of the actual crime, or an eyewitness to the ACTUAL crime. Seeing Johnny flee the crime scene would be CE if you didn't see him commit the deed.
So, CE is actually preferred to DE and it's silly to ever say this is only a CE case.
Having said that, most of the evidence (though damning) on Scott Peterson was behavioral evidences really. They did have the purchase of the boat, cement, some computer logs, etc.. and him placing himself at the scene where the bodies washed ashore, but there was no real DNA evidence since he lived at the home.
This would apply to Casey as well, but with decomp in the car, Caylee's hair with the death band there as well, statements of her acknowledging the smell also goes a long way. Also, she went the "abduction route," but too many evidences from the home were found with the remains. She too has a LOT of behavior issues and just like with Scott Peterson, it's going to be a huge downfall for the defense. But in the end, what we know right now, they already have 10X more on Casey than they ever had on Scott (aside from behavior as they were pretty equal in that dept.).