There is a reason why Sparda has the same face as Dante&Vergil. His design was to be Dante at first after all.
When they decided that their RE4 was going to become a new game called DMC they had a different story that the one we know. The game was going to be cut in two. In the 1st we had the arrogant/cocky/cool Dante we all know and love. Somewhere in the middle of the game something was supposed to happen to him and make him decide he will be more serious about the whole slaying devils thing.
The 2nd part of the game was years after the 1st part. In this Dante has matured, redesigned his weapons and clothes of choice and become more serious.
However that script was not so appealing to the other makers and they changed it.
However the design of the future Dante was already made, so they created the Legendary Dark Knight mode to present this guy as Sparda, and thus now that form is Sparda 100%.