it's situationally dependent, but I'll answer depending on what you want it for; it's all about strategy. First of all think; am I indending to use this in double battles? double battles are very rare in Pokemon main-games, less popular in competitive, and present in the battle tree, but I don't find myself using them. gardevoir's hidden ability, telepathy, prevents friendly fire from moves such as surf and earthquake. if you don't plan to use this Pokemon specifically for double battles alongside a splash-damage-move-running Pokemon, I'd recommend evolving into gallade. by the way, is it male? because if not, you don't have a choice.
But this might be the best choice: make the most of the hidden ability and don't evolve yet, 'cause there's a way to choose both. before evolving anything, breed with a ditto, and get a lot of ralts. each one has a 60% chance to inherit gallade's hidden ability. I know this from my own knowledge and experience, but for the most information on this, check the DB's dedicated breeding page. it's a good idea to get as many HA ralts as possible, but as long as you breed at least one (in addition to the one you already have) you can get both HA gardevoir and HA gallade.
come to think of it, the ability to get both kind of defeats the point of the first paragraph.