Come down on one side of the fence or the other
Definition 1 of 1
Pro Tip 1/3Decision Pressure
This phrase often implies a sense of urgency to make a clear choice. 
The boss said I have to come down on one side of the fence or the other by tomorrow.
Pro Tip 2/3Neutral No More
Use this when neutrality or indecision is no longer acceptable. 
In the debate, you must come down on one side of the fence or the other.
Pro Tip 3/3Emotionally Charged
Choosing a side can be an emotional action, expressing strong beliefs. 
After the argument, she came down on one side of the fence or the other.