I was one of the posters who mentioned it. Perhaps others did so, too.
I think the difference in planning comes from the difference in the goals.
Chris Watts did preplan, but I doubt he was in a rational mind doing it. Even if it were not for NUA, he'd be caught soon, he was just off, in so many ways. I always thought he was in a hurry to annihilate because he felt that in a week, this "eclipse"' in his mind would have been gone. He was obsessed by love, and maybe felt trapped, and it made him dangerous to a lot of people, not only his family. But the planning phase was short.
I suspect, and have no proof, that maybe PF thought of killing KB before CW case, but as to fine-tuning, the details, CW's case made a difference. He chose the day when everyone will be away of busy, he had enough time to clean, he created an alibi that bought him a week, he gave no interviews. He hid the body too well. He got rid of the purse, the cellphone, all things incriminating for CW. He did not think of the car, or maybe he knew that a car could be GPS-tracked.
The biggest mistake was conspiring. As long as a secret it known to two, it stops being a secret. Enter BFFs, their bosses, and maybe a lot of locals we don't know about.
What really played for PF was the size of the ranch, the date and the fact that his are not densely inhabited places.
But why do I think he planned it before August? Watts case unraveled so rapidly, one would think, "darn, it is difficult to kill these days, I'd better not". Only if you already have the plan in mind, you start thinking, how do I avoid mistakes?
I think that he approached KK in September specifically because this is when Watts case, the details, became known. Maybe he planned to kill KB by himself, and in September, thought he needed some "corrections".
In short. CW - love-obsessed and very impulsive.
PF - very "stuck in ideas", a planner. MOO.