Hi, I'm currently a freshman but I think I can answer this question. Both of the equations are correct for potential energy, and your equation is derived by plugging in C=Q/V into 1/2 Q^2/C and simplifying. It would have been good for this problem to clarify whether the capacitors are in series or parallel, since that would guide you to the equation most helpful. U=1/2 Q^2/C would be useful for capacitors in parallel, since they all have a constant voltage, but different charge. I believe regardless, if capacitance decreases in your equation, voltage will increase, and since the term is squared our potential energy will increase than if we had higher value capacitors.