I currently have a 2x2 table of proportions for treatment effects that looks like:

\begin{array}{lll}
 & \text{Treatment } A & \text{Without Treatment } A \\
\text{Treatment } B & 15/250 & 8/250 \\
\text{Without Treatment } B & 5/250 & 
\end{array}

I am trying to test to see if there is a synergistic effect of Treatment A and B. In this case, would a chi-square test work best or would a $2^2$ factorial design work?