in this case due to the prescence of $$\ce{OCH3}$$ $$\ce{O}$$ will act as an electron withdrawing group as well as electron donating (resonace) whereas $$\ce{CH3}$$ group will only donate it's electron (resonance) thus para-methoxybenzaldehyde should be more reactive. Again, the answer in my book states para-methylbenzaldehyde is more reactive.