depends on what you are looking for in a headphone really.
the ad700x has a much larger soundstage.
the hd558 will have a bit more bass.
the ad700x uses an odd wing design. some love it, some hate it. the hd558 uses a traditional headband.
the ad700x will be brighter in treble.
the hd558 is easy listening, though as a consequence is a bit slow. the ad700x is a bit more aggressive.
both are good choices. if you play mostly fps and want advantage, the bass-light nature of the ad700x is best for that.