I am not expert who can break down any key points or a point, at which it is imparative for you to castle to one side or the other. I would simply say that, even without calculating anything, from a strategy stand point, if you can see any sort of positional advantage, that gives you better attacking chances with a pawn storm, your minor and your major pieces, while being able to hold the fort together, going queenside could certainly be an advantage.
I would say most players who try to memorize lines from openings/defenses, of the more popular, viable lines, such as the Caro-Kann, most likely concentrate on kingside castling lines foremost, because these positions seem to be more often played. This could possibly level the playing field. While watching two of the best players in the world play each other. I noticed they both tried to throw everything, including the kitchen sink at each other.
To try to answer a few of your secondary questions. Kingside castling is generally easier to obtain quicker. This is often the reason it is chosen. Keep in mind, you do have opportunities to try to steer the game to either direction for different reasons. You could do it to try to throw your opponent off, perhaps to make em guess what you are up to. I like to leave myself options personally.
I find, with each choice that your opponent makes, they give you something you can use, or something in their position, that can be exploited somehow. Try to learn how to see this to help you make your determination. Try to take what your opponent gives you, in addition to to trying to force them to relinquish what you want to take.
If possible, make all of you moves have two fold consequences for your opponent. Don't give them one thing to defy. It forces them to use all of their resources for defense, instead of offense. Once they are defensive, their options will help you to determine how to attack for mate.
In the classic caro-kann I see games where black castles queenside and kingside both fairly common. what are the advantages of each of these and which should generally be better? do they each have unique plans?