K so lets take a look at their stats
Gardevoir - 68/65/65/125/115/80 (HP, Atk, Def, SAtk, SDef, Spe)
Porygon-Z - 85/80/70/135/70/90 (Same order)
So you can see Porygon maintains higher stats in HP, Attack ,defense, Special Attack and Speed all by over base 10. It only loses in Special Defense, in which Gardevoir holds a much better stat.
Gardevoir has abilities in Synchronise, Trace and Telepathy. Telepathy is fairy useless unless in double battles, Trace can be good and Synchronise trolls status users. On the other hand, Porygon-Z has offensive based abilities in Download, Adaptability and Analytic - with Download increasing one of it's offensive stats in accordence to the opponent's defensive stat, Adaptability making the STAB modifier x2 and Analytic increasing the power of moves by 30% if moving last.
Movepoolwise, I think Porygon-Z has a more colourful movepool than Gardevoir, as well as access to nice moves like Nasty Plot, Agility and a ridiculously powerful tri-attack.
Anyway so I'd probably choost Porygon-Z over Gardevoir in most cases (Gen 5 and under) due to better overall bulk, speed and special attack. However there is a case where I would back Gardevoir - if you are referring to Gen 6 (Pokemon X/Y) I'd back Gardevoir if you had access to it's mega stone, so you could Mega Evolve it. Mega Gardevoir > Porygon Z. :P