How are these spells different exactly? do they do the same damage?

They all hit the same, but with different side effects:

 

 

 

 

 

 

 

Saradomin causes opponent's prayer to be temporarily lowered.

 

 

 

 

 

 

 

Guthix causes opponents' defence to be temporarily lowered.

 

 

 

 

 

 

 

Zamorak causes opponent's magic to be temporarily lowered.

 

 

 

 

 

 

 

It's all preference, I suppose... they all hit a max of 20, or 30 with the Charge spell.

nerdattack16 is right about the 'specials' but u'll hit max 20 normal, 25 whit wearing the god cape at the same time and 30 whit god cape and charge