I lean more towards the 'off-the-cuff' approach, having long ago learned that:
" No Adventure Plan Survives Contact With The Players "
AND:
"The amount of time, dedication, work and detail put into adventure preparation by the GM/Referee, is inversely proportional to the likelyhood that the players actually grasp at ANY of the plot hooks FOR that adventure."
In practice, I try to have several (fleshed out) plot paths out there for the PCs to choose from at any given time. Each path has a definite direction, but has a generous amount of "wiggle room" to allow for the 'Player Choas Theory'.
-- and also have plenty of hints, encounters and rumors to steer/entice/trick them back to one of the established plots.
Generally, I just like to have a cache of scenes/nuggets/bits (whatever) that all tie somewhat to one of the main storylines I have in place. As needed, I can drop different pieces in place (mixed in with neutral or red herring varieties) depending on player direction.
Just so long as, ultimately, the path they choose is there own.
(or at least appears to be)
In my experience, nothing poisons a game's fun, more than the players believing they are railroaded down a set script, with little or no chance to effect the outcome.
Despasian