Gardevoir > Diancie
It's a tough choice since their both gaining something they need.
- Less weaknesses
- A great ability (since he can learn strong normal-type moves)
- Wide Range of moves and various types
- excels in sp. attack/defense and speed.
- could use an extra boost of attack or defense to make her a true sweeper or wall
- low HP
- better moves are based against TM's rather than evolution moves
- low base stat total
- high base stat total
- excels in all categories and could make a great sweeper or wall
- can learn strong moves to increase STAB
- good ability
- More weaknesses
- doesn't have much of a wide move category
With Mega Diancie, you boost it's speed (necessary), but you still need the HP. But, regular Gardevoir doesn't have much either. If you give Diancie the mega, it will be better if you go for a strong sweeper and wall. With Mega Gardevoir, you basically just strengthen what he's already good at. You should choose this mega to equalize out both Pokemon's stats, to make them fairly similar and strong. Seeing that his base stat is much lower, he would definitely need it.
Both have good abilities and any one would be good. In my opinion, Diancie would be better if you go for a solo attacker and defender. But, mega gardevoir would be better if your going for double/triple battles or anything team related. But since your having both on your team, definitely choose Gardevoir - and give Diancie Leftovers/Shell bell to really deal with his low class HP.
Your choice! Hope I helped! :)