It Depends.
Normally, psychic would be better since it's the stronger Sp. attack move. But, psyshock can also be used differently.
Psyshock uses your special attack stat but hits into the enemies physical defense stat. Psychic also uses your special attack stat but hits into the enemies special defense stat.
I've found it is down to preference really, both can be useful. Psyshock is also a great way to give Pokemon like Latios/Cresselia who are special based a good way to hit opposing Pokemon in their weakest defense stat, most notably 252HP/252Sp.Def Amoonguss.
Psychic is pretty powerful when you just want a strong special sweeper to attack with.
I would use psychic, but not necessarily
Source
Hope I helped! :)
