Well, usually the concept of equal characters is that one's "stronger" and the other is "faster", as this is the case in Street Fighter II.
In DMC3, I believe this sticks, Vergil is much faster than Dante, but in my opinion, Dante is slightly stronger, he also has more options in weapons and styles.
Also, Vergil has no cutscenes, accept the intro...Why Capcom? Why make an intro but no in-game cutscenes?
I think Vergil is in a sense, more "badass", if you wanna go by his violence and attitude, but Dante is supposed to be the main hero, and the funny, bigheaded one. People don't like him in DMC2, but you gotta remember, he was much younger in DMC3, people mature as they get older, as I think Dante did in this case.