In my opinion they're both masterpieces, but C;C is better because of the plot and the characters.
I would recommend you to play Chaos;Head before Chaos;Child since playing Chaos;Head can give you a better understanding of the universe of Chaos;Child.
As for Steins;Gate you can play it before or after, it changes almost nothing, this because even though they're set on the same universe, they don't have much connections.
In my opinion they're both masterpieces, but C;C is better because of the plot and the characters. I would recommend you to play Chaos;Head before Chaos;Child since playing Chaos;Head can give you a better understanding of the universe of Chaos;Child. As for Steins;Gate you can play it before or after, it changes almost nothing, this because even though they're set on the same universe, they don't have much connections.