Well, they both have their merits. I won't restate everything Muguet said, as I'm sure their answer has been read.
Even in X and Y, Charizard, Greninja and Heliolisk are most often seen offensive. Goodra and Aegislash are often bulky or tank like. I personally like the idea of a third bulkier defensive mon, over a more offensive style, given the current team you listed.
Factoring in stats is important, (as Muguet did) but so is move pools and abilities (which Muguet didn't). Florges has no relevant ability in singles competitively or casual. Symbiosis is useless in singles. Sylveon gets Pixelate (Hidden Ability) and Cute Charm (Normal ability). Sylveon is clearly better ability wise.
Sylveon is always bulkier than Florges, which is important. It can take hits a lot better, and set up it's own moves easier. (See my suggested set)
They both get Moonblast and Calm Mind, and Wish and a Cleric like move (Heal Bell or Aromatherapy), so no clear winner there.
However, Pixelate grants a 20% boost to a normal type move, and Sylveon learns both Hyper Beam, and Swift. Sylveon also gets STAB from those moves. Thus making it's special attacking damage output equal to or greater than Florges, even though base stat wise, Florges's stat is +10.
As you mentioned it was in game only, Pixelate may work for you, if you're happy to run Swift and/or Hyper Beam. If you can't get Pixelate (Hidden Ability) or don't like available normal type moves, Cute Charm is the only option, and you can use Moonblast. I've personally found great use in a Sylveon like this:
252 HP / 252 Def / 4 Sp Atk
Requires Calm Mind/Wish set up, but can be an absolute powerhouse in game, especially if ev trained.
You can swap Cute Charm for Pixelate, and Moonblast for Swift. Personal preference.