Both are good for different reasons. KD had exponentially more rides than BG, and a bigger collection of thrill rides that might appeal better to a 12 and 14 year old. Cedar Point has been pumping in cash to restore KD to is pre-Viacom glory, but it has always been the "riders" park of the two, but is more carnival in atmosphere. It's easy for thrill-seekers to finish every thrill ride in BG before lunch in a moderate crowd.
BG is a nicer laid out park and appeals more to adults who prefer a more leisurely pace. And it has some great coasters as well. It's beautiful and better shaded, but requires treading up some hills. I have to say the Europe theme is getting a little old in the tooth and tacky.
Food sucks at both but historically was usually worse at BG, when Anheuser Busch owned the parks, they pretty much served airline food. BBQ restaurants at both parks are saving graces.
Beyond the rides, KD's built-in water park is pretty lame compared to Water Country.
As I noted both are good parks. If killing one day, I will always pick KD, if you want to stay for a few days, Williamsburg and BG offers more to do and better hotels.
If you want dirty and gross...that's what Six Flags is for.