Which is more acidic: p-xylene or 3-methoxytoluene?
$\text{Acidic Strength}\propto \text{-I, -M, Negative Hyperconjugation effects} \propto \dfrac{1}{\text{+I, +M, +Hyperconjugation effect}}$

Recall that $\ce{-OCH3}$ is a moderately activating group as its mesomeric effect overrules its inductive electron withdrawing effect. 

On the other hand, you must be aware that $\ce{-CH3}$ is only weakly activating because of hyper conjugation and inductive electron releasing effect. 

Now, its easy to conclude that a strongly activating group would result in more electron density over and around the benzene ring and hence destabilise the anion formed after deprotonation. Thus, we'd expect p-methoxytoluene to be less acidic than p-xylene.