It will be hard for you to get a really good answer on this question, as most people have not skied both skis. They will probably recommend whichever one they have skied.
That said, I have not skied either, but have talked to people who have skied both. The ON3P is significantly stiffer hand flexing, and sounds like it grips snow better when you put it on edge. Both seem to do a really good job at smoothing out the ride, I have not heard of either chattering. The K2's are going to bend easier, allowing you to butter them very easily.
Both are good options for and all mountain ski. Personally I would go for the ON3P's since I prefer a stiffer ski. You can also get them on pre-sale right now.