(I'll assume you mean for a level 100 pokemon.) Psychic will do a set amount of damage each time, and for a pokemon with a high special attack stat, this will be good. Let's not foget with Dark and Steel types aside, Psychic moves work on just about everything to a normal or super effective range. Now, we look at a Psywave, which can range from 50 damage to 150. This may seem good, as 150 is much higher than Psychic's set level (85 I think, correct me if I am wrong.) on a glance. But since the chance is random, that is not really something you can rely on. I for one would choose the consistance of Pyschic, as you can learn when or when it will not work, where as Psywave it's a gamble, and thus not worth it, if you ask me. Damage is nothing if it's not able to hit. (I have the same ideals with accuracy, but that's different topic material.)