yea it's the Special attacks that make the difference, BGS and AGS have the same Spec I think, but AGS only uses 50 % spec bar & BGS uses 100 %, SGS spec makes u recover the HP u hit, lets say you hit 500 you will be healed 500 HP. ZGS spec freezes the enemy ( like barraging ).
Bandos Godsword lowers their defence level depending on what you hit.
Zamorak Godsword will freeze them (use to be an Ice Barrage until a recent update)
Saradomin Godsword will restore your Constitution and Prayer points depending on what you hit.
Armadyl Godsword - I don't know what this does but it just hits as hard as fuck.