As always, research is key here. Do as much research as you can into kirlia’s evolution, using the internet, opinions from trainers who own gardevoir or gallade themselves, and guidance from your pokédex or regional professor. Share all of the information you gather with your kirlia at each stage of your investigation, and let him decide what feels best for his identity, based on what you’ve told him.
I’d like to refrain from giving you too many specifics because I don’t want to sway your decision, but I will say that, of course, gardevoir is the more psychic evolution, while gallade is the more physical. Both stand on equal footing, as both fulfill their respective roles reasonably well. It’s just that gallade might be slightly more difficult to obtain due to the rarity of dawn stone, but that doesn’t mean he wouldn’t be worthwhile to train (or that he’s more desirable than gardevoir, for that matter).
Best of luck, anonymous, but one last piece of advice: give your kirlia an everstone until he’s ready to evolve. He needs to focus on his decision, not on resisting evolution, and the effort put into resisting evolution may either put him off the idea of being a gardevoir or make him feel like he needs to give up and become one, depending on his personality.
The Pokédex According to Bill 