I'm with everyone; trial and error. You learn to use what works for you. I have a regular wisk AND one of those I was raised cooking with -- it looks like a spring that is loosely coiled at the pan end, tightly coiled at the handle end. I wouldn't dream of making gravy without the latter, which is a very 60s kind of untensil. I honestly don't know anyone who still uses the latter, but someone must, because they are still there at the grocery store. On the other hand, the traditional whisk is perfect for light batters, whipping cream, etc. Sometimes, too, it depends on who is doing the cleaning. If I'm doing the cleaning, and am simply stirring a batter, you'd better believe I'll use the spoon rather than the whisk, especially if it is likely to be sitting on the counter 'til after dinner.
As a general rule, I consider the whisk we're really all talking about here to be best when you want to get some air into the mixture. For plain (slower) stirring, a spoon.