That's actually a good question. But since the Summoned Swords are used by many characters in DMC (Nero, Credo, Vergil, Dante) and they aren't linked to a specific item, it can be considered as some form of demonic magic.
As for who wields the swords better, Vergil and Dante both got different style with those. Dante have more control on the swords, but seems to lack the ability to control them at long range or summons them from far away. Vergil on the other hand seems able to summon those swords from far away, even directly around the target when he is flying 50 meters away from it, but he can't control their movement as precisely as Dante.
The only problem I got with those swords is why Dante didn't used them before getting his own devil sword.