Decisiveness seems hardly the word to distinguish J from P. P seem awfully decisive about many things, so then you get off-hand explanations: P can be decisive about something, if ..
Flexibility? I'm not that much in favor of that word to describe P anymore. J can become uncomfortable with the progress and the general mood of a project, but endure anyway. Now that's flexibility, just other kind than it is to "flex" elsewhere.
Process- or result orientation? J care about the end result, P about the process. This seems a bit more like it. I don't like how this is generally explained tho; (edit: by the common stereotype,) J marshal their way to victory, damned if anyone steps in the way! P are wishy-washy feel-gooders, smelling the flowers while not necessarily accomplishing anyway. (edit: disclaimer: I am not, and I was not in favor of this interpretation of J/P behavior.)
I guess we know the goods / bads of each, but I'd just like to form a good explanation of it, something that gives a balanced view of the subject in perhaps 2-4 sentences. We could form this explanation together and put in a FAQ post along with some other FAQ-worthy questions.