well we just learned about this in culinary school, so here goes...
Creaming (basically whipping the butter and sugar together) is what you are supposed to do when mixing a cake with room temp or softened fat used in it. You are actually supposed to incorporate a lot of air into the sugar and fat as is a part of the levening process. (so the short answer to your question is: Yes that it makes it lighter). The whisk is ideal for this. If your batter isn't to heavy then it's fine to use for the whole thing.
That was probably WAY more info than you needed...but I have a test on it in the morning, SO I just used you to practice on