In general, "more preparation" does not equal better. It takes a spectacular combination of GM personality and GM competence matched with particular expertise, matched with the absolute perfect group of players, in order to make a "high preparation" game worthwhile; or indeed a very particular kind of campaign to make "high prep" necessary.
The Pundit's advice to GMs is this: Prepare for your adventure only the absolute minimum that is necessary.
The more you "prep" the adventure, the more the chance that you will end up railroading or falling into the masturbatory hideousness that is storytelling. The "minimum that is necessary", however, will vary depending on which game you are running and what kind of campaign you are running with each game.
Still, "prep better", not "prep more". And what you ought to prep is the situation, the premise for the adventure, with something that outlines the basic things you really need to have happen in that adventure (which is why the Roman game is so high-prep, there are so many things that have to "happen" in each adventure), and keep the rest totally open. Its not your job to fill in the blanks, that's the players job, and they must do it however they want to.
If you're doing too much more work than your players, then chances are that you're not giving your players room to shine.
(Originally published: February 4th 2006)