Both, as was said by njgirl1999, have their own merits. You are comparing, really, apples and oranges. Both were built on the myths and legends each entail, which is really the only similarity both series has with the other.
Personally...Percy Jackson wins out since the take on the Greco-Roman mythologies have always held a special place in my heart and the take on those myths is refreshing.
Plus vampires don't sparkle. I'm an old school author who uses vampires and other undead in what I write, so the Twilight series leaves me rather cold.