Round 1: Random encounter in a bank at midnight. Both have standard equipment and neither was expecting the other, nor do they know who the other is let alone what they are capable of.
Round 2: Mysterio has had months to prepare a trap for Batman, eventually luring the dark knight to a warehouse. Batman does not know who Mysterio is and is actually expecting Scarecrow to be there. Mysterio has taken the months of prep to study Batman and his standard gear, knowing Batman has gas masks and the like.
Who wins and why? The World's Greatest Detective or The Master of Illusion?