20mm cannon was ok if you hot the target and didn't empty your magazines too quickly. .50 calibre is about 13mm and has plenty enough energy to stitch drill an enemy aircraft. Cannon fire will demolish an engine block whereas the lighter ammo may not be as effective however 'Time on the button' is increased weight for weight in the number of rounds available. Spitfire pilots were taught to be economic with ammo. Then again the BF109 had fuel injection which gives a theoretical advantage in inverted flight...well it did until those fiendishly clever Tommies figured out carbs that work upside down.