Mega Gengar - 60 HP / 65 Attack / 80 Defense / 170 Sp. Attack / 95 Sp. Defense / 130 Speed
Ability - Shadow Tag - The opponent cannot run nor switch while this Pokémon is in play.
Mega Gardevoir - 68 HP / 85 Attack / 65 Defense / 165 Sp. Attack / 135 Sp. Defense / 100 Speed
Ability - Pixilate - Increases the power of Normal-type moves by 30%. It then changes those moves to Fairy-type.
You don't have Ghost / Poison / Fairy / Psychic.
Here is a moveset for each one -
Mega Gengar @Gengarite
EVs - 252 Sp. Attack / 252 Speed / 4 Sp. Defense
- Sludge Bomb
- Dazzling Gleam
- Energy Ball
- Shadow Ball
Gardevoir @ Gardevoirite
EVs - 4 HP/ 252 Sp. Attack / 252 Sp. Defense
- Hyper Voice (Pixilate = Powerful fairy STAB)
- Calm Mind
- Wish/Shadow Ball/HP Fire/Thunder Bolt/Focus Blast
I would go for Gengar because has more Sp. Attack and Speed making it a great sweeper.
Gardevoir has great Sp. Attack and Sp. Defense.