How to predict the outcome of a crossed Cannizzaro reaction?
To answer this, first let us look at the mechanism of the Cannizzaro reaction in general. It is depicted in scheme 1 as a cyclic transition state. The cycle itself isn’t as important, the realisation that one aldehyde is present as a (deprotonated) hydrate after the action of $\ce{OH-}$ is.


Scheme 1: General mechanism of a Cannizzaro reaction.

In your reaction, you are reacting formaldehyde with a (substituted) benzaldehyde. The question boils down to ‘can we predict, which aldehyde will form a hydrate mor easily?’ And the answer is: we can!

$$\begin{array}{cc}\hline
\text{carbonyl} &amp; \text{hydrate ratio} \\ \hline
\ce{H2CO} &amp; 99~\% \\
\ce{MeCHO} &amp; 60~\% \\
\ce{Me2CO} &amp; &lt; 1~\% \\ \hline\end{array}$$

We have a clear tendency for formaldehyde to almost exclusively form hydrates under standard conditions. Thus, we can conclude that formaldehyde will alwys be the hydrate partner, end up oxidised and reduce its reaction partner as shown in scheme 2.


Scheme 2: Cannizzaro reduction of an aldehyde with formaldehyde as sacrificial aldehyde.