Scenery- KD
Rides- KD
Park layout- KI
Themed areas- KI
Food- KI
Water park- KI
Kids area- KD
Kings Dominion has some kick ass foliage, architecture (both go in scenery), along with their rides and coasters. Kings Island easily has the best park layout and themed area flow, along with food. Soak City is much better than Water Works, and the kids area stuff at Kings Dominion looks a lot more family friendly.
I've been to both parks, and overall I would say they tie. They both have things that the other doesn't, and some things in common. In a sense of trip planning for a thrill seeker, I would choose KD over KI. Though if you want more family oriented stuff (quanity over ambience) go to Kings Island.