The only difference between the pairings is having steel type coverage. I will say that steel is very useful against the fairy gym and the ice gym. However, aside from this, let's talk about their offensive stats, along with their pros and cons. I will be giving each 'mon a score out of 50, and the gardevoir + Lucario pairing gets a 10 point boost for the gym advantages.
-You have a mega stone that you get from korrina I believe, jacking the power up to a hundred. 10 points for power.
-An excellent speed stat of 112 that allows it to speed most, if not all, of the game (excluding the priority moves). 10 points for speed.
-Decent early-game stab in power-up punch + swords dance, however, not really any metal stab out of metal claw, which is learnt at level 1. + 7 points.
-Fighting-Steel is an amazing defensive typing, with only three weaknesses, fighting, fire and ground. Admittedly, those are quite common types, however, this thing boasts 8 resistances and 1 immunity. Pretty good. +9
-Its defenses, aren't very fun. 70/88/70 is al bad. +5
Overall, not counting the bonus, we have 41 points for lucario.
Next, we have gardevoir.
- Gardevoir has an amazing special attack stat in 125. +8
- Not the best speed stat in 80, however, it can work. +6
- Gardevoir has a horrible level-up movepool, only getting psychic moves, however, it has a decent stab move in psychic at level 40. + 2
- Fairy Psychic isn't the best defensive typing. It has a measly 2 resistances and 1 immunity, and the immunity is nice, but other than that, it sucks. + 2
- It has horrible HP and defense, but really good Special Defense. +6
In total, without the add on, we have 24 points.
This pairing gets 65 points, + the ten, we have a total of 75 points.
Next, let's talk about gallade and sylveon.
Gallade basically has the same stats, so the only things that I'm going to score are movepool and defensive typing.
- Gallade gets psycho cut and swords dance, making for a monsterous combination. It, however, doesn't really get good fighting stab other than close combat, which it gets at level 59. +4
- Psychic-Fighting is worse as a defensive typing, as it is psychic fairy, but without an immunity. Also, it doesn't have a 4x resist. +1
Totalling gallade, we get 25 points.
Finally, we have sylveon
- Sylveon has a decent 110 special attack stat. +7
- Sylveon has...quite a bad speed stat in 60. +4
- Sylveon has...a bad movepool too, only getting fairy moves to fire off its special attack. +2
- Pure fairy is...quite a bad typing defensively. It has 3 resistances and one immunity. It has two weaknesses. +2
- Sylveon has really good bulk. 110/65/130 has good special defense, and while the defense is bad, the HP more than makes up for it. +9
Overall, we get 24.
25 + 24 = 49.
I would choose lucario + gardevoir based off of my system.