Thanks I didn't know that they play in the same universe. I guess I will start with CHAOS;CHILD since it came out first and I read that it's a little weaker than Stein's Gate.
I didn't know either until I looked it up. Either way, it only says they're "interconnected" so maybe it doesn't really matter what order you play in so long as one isn't a direct sequel of another
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.