Calm Mind, all the way.
Gardevoir has excellent SpD (115) and even better SpA (125), having a move to boost both of those stats is important, plus Gardevoir's role isnt setting up screens, it's a bulk-ish special sweeper.
Also, Gardevoir's Def stat is so low you just simply dont send it against Physical attackers, unless you know you'll outspeed and OHKO.
If you want a good screen-setter, use this Clefable:
Clefable @ Leftovers
EVs: 252 HP / 4 SpA / 252 SpD
- Light Screen