As a guy who uses Espeon I would say go with Gardevoir, Gardevoir has a better typing, it also has a better movepool getting access to calm mind and moon blast give it good coverage and sweeping potential, ability wise Espeon would normally win since magic bounce is incredibly powerful but in reborn very few trainers actually use status moves I've carried around my Espeon for a long time and I've only seen magic bounce proc around 10 times while trace though kind of random is a very good ability, stat wise I think Espeon has a little more sp atk and is a bit faster while Gardevoir has more sp def. Overall I would go with Gardevoir, its a bit more flexible than Espeon being able to sweep with calm mid, has better coverage, and its abilitly is a little better, theres a reason why she go moved so far back in the game.