Naturally, I would advise you to choose depending on your Medicham's offensive stats: Zen Headbutt if it's Attack is higher, Psychic if it's Sp. Attack is higher. However, I'm going to assume your Medicham has Pure Power as it's ability, making Zen Headbutt a much more powerful move.
Consider if you want to prioritize power over accuracy! If Medicham's Sp. Attack is superior, I would just stick with Psychic.
Another thing to consider is if you want your Medicham to be a mixed attacker or specialize in one category. If it specializes in either physical or special moves, you may want to stick with that or keep it mixed, again depending on it's stats. (It would still benefit from Pure Power if it is a mixed attacker)
Also - the flinching that Zen Headbutt causes may really come in handy if your Medicham has high enough speed. Does it? Consider it's stats.
Sorry if I rambled :) I hope this helps!