Gardevoir learns Hyper Voice via move tutor in ORAS (and B2W2), which is the main reason to use Mega-Gardevoir, because Hyper Voice will receive STAB from Pixilate, and also bypass substitute.
However, Mega-Gardevoir is still relatively slow, at only base 100 speed, so realistically it needs some Sticky Web support.
Regular Gardevoir has the benefit of being able to hold Scarf, but lacks the sp. atk buff that M-Gardevoir gets, so it still ultimately depends on your team to decide which one to use.