Which is better of the two
Sylveon's major stats are Sp. Attack and Sp. Defense, while the other stats are quite below average.
Gardevoir's major stats are Sp. Attack and Sp. Defense, so they are two very similar Pokemon.
There Attack and Defense stats are the same, while Sylveon's speed is lower than Gardevoir's. The truth is if you want a defensive Pokemon, go for Sylveon. If you want an attacker, choose Gardevoir.
Also, Gardevoir does have a Mega-Evolution, but since the stone is only available post-game the Mega Evolution's helpfulness will only last post-game.
If I were to choose, I'd choose Gardevoir because of how easy it is to evolve, because you have to use Pokemon Amie, which is a pain if you don't normally use it. (I do though. x3)
Really, what you want to choose is absolutely your option.
What level will ralts from Diantha be.
Level 5. It is only available post-game, and it will be holding the Gardevoirite.
She'll trade it for anything.
What are the odds of getting a female Eeveeon route 10 I keep getting boys.
Meloetta is correct, but just to reiterate it's a 12.5% chance it will be female, while the latter will be male. That's a one in eight chance if your not good at math.
I honestly can't imagine a Male Sylveon. With the bow, and the ribbons, and the... O_O
Hope I helped. :)
Source, Source, Source and my brain. :D