Are ye an organizer or last minute?

Something I have noticed is there are really three types of GMs when it comes to session planning. There are those that write out everything and have it all ready for copy/paste into the session as it goes on (Sometimes even having it as a script, so all they have to do is type in a code and IRC pastes it in for ye.) or there are those that wait till last minute and do most of the session off the tip of their hats, just BS'ing along until they get the few points they were able to get done before the session started. There is also a third type, one that does a bit of both. Writing out little bits that intro an enemy or scene, and BS when things go off the beaten path and until things get back to where they have things written down.
So what are ye? Plan everything beforehand, Put it off till the end and BS most of it, or Plan some BS other.

Personally I'm more of the Plan everything beforehand, but I know I can't plan for everything so I expect my players to put me into the BS category from time to time.

In Demon-World, for instance, I've planned out to have about 100 main-stream missions (so the basic, core story) as well as tons of Base Defense (as in, random fight if I see a bunch of players online and we have about an hour or so to have fun) or Training Missions (Mini 2-3 hour session just to pass the time or get into character development instead of story development.
Out of those 100 missions, I have about... 60 of them planned out (not written out yet, but I know what I want to happen) and of those 60 I have about 20 written out already (I love to stay well ahead of the story, that way I can make changes down the road and it seems it was supposed to be like that from the beginning. Not like "oh Shi... I need to rewrite the next mission because xyz happened.")

Hope this sheds some light on the mind of a GM.