The extra power is almost always better than the flinch chance, supported by the fact psychic also has its own not bad secondary effect.
The only Pokemon that may use extrasensory over psychic would be Pokemon with the ability serene grace, like togekiss. serene grace doubles the chance of secondary effects, so repeatedly flinching the opponent is better than not doing that with psychic, without serene grace the chance is too small for extrasensory to be better.
Hope this helps.