Pros
Gardevoir has better decent speed and high Special stats. And gardevoir has a reliable move to cover dark types signal beam.
Gochiruzeru on the other hand has highier defense and hp.
Cons
Despite being one of the best special tanks around Gardevoir suffers from a low defense stat and a low hp stat along with an average speed stat. Though aside from this Gardevoir is great.
Gochiruzeru also has a low though not as low hp stat along with lower speed, and Special stats. but Gochiruzeru is still a great pokemon.
so if you want more damage inflicted go with Gardevoir. If you want more damage taken go with Gochiruzeru.
Note: both work great with calm mind