Since the thread maker clarified that he insists on discussing this taking into account that Sparda is a costume for Dante then I'll vote.
Definitely Sparda. First of all, in gameplay he has more weapons, better Devil Trigger (I swear, Nero's Devil Trigger only heals you and lets you deal more damage, no greater speed, no power to stand your ground when enemies hit you etc etc), the Styles - which give you far more awesome opportunities and abilities to outsmart your enemies during gameplay (first of all all Styles have many moves whereas the Devil Bringer only has 3 - and to my personal opinion, the move in which Nero uses enemies as a shield is almost worthless
yeah I'm biased), and not to mention who looks cooler during gameplay. While playing we can see Sparda's demonic shadow even when he's in human form - showing us th difference in how awesome a true demon is in comparison to half-demons and humans who only have a bit of a demon blood running in their veins.
Nero's gameplay is more like: attack->evade->shoot->snatch->devil bringer->repeat with little variations.
Sparda's (well Dante's) gameplay is more like:
attack->style->another style->gun->change weapon and attack again->and many many many more variations on this.
To be honest, even Vergil who only had one style, could allow for more variation during gameplay making it all far more interesting.
So for all these I vote for Sparda (Dante's costume anyway). Even if he hadn't Unlimited DT, he outranks Nero in every regard except maybe the fanbase in which Nero seems to have the upper hand given the fact that we have yet to see Sparda appearing in more than just a Dante skin.
