Do you have an external series of events that are impacting your main characters? Are they pursuing their own objectives, or are they only reacting to the situation they find themselves in? Are your characters always on the defensive? What do you find yourself focusing more on, the plot or how the characters are interacting with each other?
Some start with strong characters and build the plot around, letting the character motivations drive the action. Other times, there can be a rather simple main plot running through the entire piece, but then there are separate actions the characters take that impact that main plot. Sometimes, it can even happen that a character action, to be authentic, requires you to adjust the main plot for it to still make sense.
The choice can also come down to the kind of story you are trying to tell. A technothriller, where the protagonist is tracking down a terrorist cell, is going to have a much stronger central plot than a fantasy story where the main characters are a group of adventurers making their way in the world. Yes, a balance needs to be struck, but the type of story you are writing is going to strongly influence how heavily weighted one side is over the other.