But in DMC3 SE, did we actually see either of them in Sparda form, I only remember that *** Arkham trying to take Sparda's form.
And I think its only natural for Dante and Vergil to take the true form of their demon side as Sparda, they are his children afterall. As for taking a form thats similar, I think that would work also, but the DT form has to be similar or even more menacing