Formaldehyde has a H- (Hydrogen) attached to the Carbonyl carbon of -CHO (aldehyde) group.
Whereas , Benzaldehyde has a Ph- (Phenyl) group attached.
The major reactions of aldehydes are Nucleophilic addition reactions(NAR)[ in which a nucleophile gets added to the Carbonyl carbon which is electropositive in nature].
Addition of a Nucleophile depends on Steric Factor.
The less sterically hindered the Carbonyl carbon is , greater is the ease of addition of Nucleophile.
In case of Benzaldehyde and Formaldehyde , Benzaldehyde has a more sterically hindered carbonyl carbon (due to bulkier Ph- group) as compared to Formaldehyde (with a smaller H- group). Thus , Nucleophilic reactions takes place faster in case of Formaldehyde.
Therefore , Formaldehyde is MORE REACTIVE TOWARDS NUCLEOPHILIC ADDITION REACTIONS than Benzaldehyde.