Both are valid issues however I think the issue of preparation time can be resolved (or at least mitigated) by developing a processes to help speed up the time it takes to turn the idea bouncing around in your head into usable story material.
One of the biggest aids to me I just discovered recently. It is called a mind map. It basically diagram that starts with a word or phase with branches for major categories composed of words or phases related to the idea. These major categories are themselves branched out from by smaller sub categories.
Once you have these ideas you probably will move things around and better categorize them which will help you to better organize those thoughts. Hopefully in the end you have turned your collection of ideas into something more structured and useable.
Here is an example of an adventure I made using the idea of a story based around the song "I shot the sherif." I used coggle.it history feature to show the progression of the mind map.
I have found this very useful in helping me put together adventures. By virtue of going through the process even my most mundane ideas have turned out to be more complex then they would otherwise.
Thanks for reading!