Why SO3 is more acidic than N2O5 besides N being more electronegative than S??

From the Lewis acid base concept , a base is defined as a substance which can furnish a pair of electrons to form a coordinate bond whereas an acid is a substance which can accept a pair of electron.

Therefore acidic strength can be compared in terms of the ability of electron acceptance.This said, as we know that the electrons are negatively charged, oxide of an element with more positive oxidation state will be more acidic. You can compare the acidic strength of various inorganic oxides by comparing the oxidisation state of the respective element in their oxides.

For instance, considering the compounds you suggested,

Oxidation state of S in SO3 is +6

Oxidation state of N in N205 is +5

Since S03 has S with more positive oxidation state than N in N205 , SO3 is more acidic.