Empoleon vs. Magnezone
Since you want a defensive Pokemon, lets take a look at their stats!
Empoleon: 84 HP/88 Def/101 Sp. Def
Magnezone: 70 HP/115 Def/90 Sp. Def
Obviously, Empoleon has better HP and Sp. Def than Magnezone. Magnezone, however, excels in Def. Empoleon has more even stats and might be your first choice, but it is important to note that Magnezone beats it by 27 base Def stats and has good Sp. Def/decent HP. To determine it, we would need to look at Gardevoir's stats.
Gardevoir: 68 HP/65 Def/115 Sp. Def
Because Gardevoir has great Sp. Def already, I would suggest Magnezone because it would help cover Gardevoir's frail Defenses. But, if you have a Magneton and cannot evolve it, Empoleon might be better.
If you can't decide, you can look at these websites:
I hope this helped!