Canned adventures redux

So I went ahead and tried the whole "create a canned adventure" for the Session we are currently doing. I mean I basically had a good idea of where I wanted the adventure to go and what not, so I started typing it up. On the good side, it really helped me design a quest that used the strengths and weaknesses of the chars that are currently playing. However even with spending as much time as I did planning out all the possible outcomes, they went and did something I didn't even think about. Here is where I think the "canned" adventures throw me. When your GM'ing and you are doing it off of a vague idea, or a basic set of notes, you are pretty much prepared for anything that the chars throw out there, because you are not expecting them to do anything. However when you write up the way the story "should" unfold, if they don't stick to the (your) plan, it is easy to feel like a deer in headlights. That being said, I think I'm going to continue to create notes and basic ideas of how the quest should go, and layout combat (create monsters, villains, ect), but try not to detail every little thing, and have the chars do what they want and fit the story to them. Because after all, the story is about them, I just provide the backdrop :)

pregenerated adventures vs. top of the head adventures

So in the realm of RPGs there are two main times of GM'ing:
1) PreGenerated Adventures aka "canned"
2) Top of the head Adventures

So which one is better? Coming from a Palladium Fantasy background myself, we always did top fo the head GM'ing. Usually our quests revolved around whatever our current favorite movie was, or something we saw in the book and wanted to throw at the players. We only tried a couple of the "canned" adventures, and they were always way to complicated to navigate the pages, and it seemed to slow down the story. So I used to be very against them. Anyway, I have since started listening to rpg podcats, from penny-arcade, as well as rpgmp3, and I have noticed that they are using "canned" adventures. I have to say, that they adventure itself moves very fulid and the story is really good. So I though to myself, maybe I just wasn't ready for "canned" adventures when I was young. So I found myself an adventure ( actually a free DnD 3.5 adventure, and I'm going to mix it into my current campaign and see how it goes. ( currently converting the stats and items over from DnD rules to palladium rules :). So I'll post more feedback on my thoughts about which one is better and why, when I finish.