With over 150 hours (+200 if you count soft resets) clocked, I've played my fair share of Pokemon Y.
Now that I've gotten a good stab at competive battling I'm starting to wish I had gotten X.
What is to follow is my opinion and is in no way fact.
With the signature legendaries Yveltel has a good Bulky / Support role but Xerneas is an almost broken monster. To start with Yveltel, Dark/Flying is quite good offensively and he has 99 base speed which outspeeds the common Bulky 90 base Speed Legendaries (made better cause he knows Taunt). His Signature move Oblivion Wing not only hits hard but heals as well for offensive sets, otherwise Roost for defensive sets. But then comes Xerneas with its signature move Geomancy. Geomancy with the item White Herb (so it doesn't take 2 turns) is like Quiver Dance on steroids, +2 boost to Special Attack + Defense and Speed. This basically makes him the strongest sweeper in the game minus Extreme Speed Arceus and Kyurem-W. This is one of the few superior choices where I'd rather have Yveltel cause of the OP'ness of Xerneas.
Mewtwo Y really only has speed and Psystrike, with Mewtwo X you gain its new Psychic/Fighting type (yummy Drain Punch or Low Kick against super fat legendaries) and is more bulky overall. Mewtwo X can also run a mixed set just fine.
Before the knowledge of the Weather nerf, Charizard Y looked to over shadow Charizard X, but with better stats for a Mixed Attacker, a boosting move like Dragon Dance/Belly Drum, and a lessened Stealth Rock weakness Charizard X is currently a better choice for your Mega slot.
The rest of the Megas stones are found in one game and the pokemon in the other. But of course who's gonna trade their stone away. Hopefully event pokes will allow you to get the other versions stone (and with the leaked Mega Latios/Latias reveal this looks likely).
Y gets the Mega Stones for Aggron, Houndoom, and Heracross. Mega Aggron becomes an insane defensive tank with Filter (super effective moves do 25% less damage) but lacks reliable recovery and bad Special Defense. Mega Heracross gets Skill Link so Attacks like Pin Missile and Rock Blast always hits 5 times and gains some bulk but loses speed and is hard to want Normal heracross with Guts (if burned/posioned = +50% attack) or Moxie (+1 attack boost per pokemon fainted). Houndoom gains bulk + speed and gets Solar Power (+50% Special Attack in the Sun at the cost of 1/8 health a turn) but of course there is the problem of the Weather nerf limiting its potential.
X gets Mega Tyranitar, Pinsir, and Manectric. Mega Tyranitar is a god among pokes with amazing attack and bulk (sandstorm boosting that Special Defense) with only speed being its downfall (which Dragon Dance can potentially fix). Mega Pinser gets Aerilate (boosts normal moves by 33% and makes them flying type) which makes Double-Edge one of the hardest hitting moves in the game, he doesn't like the x4 weakness to Stealth Rock tho. Finally Mega Manectric becomes a speedy Special Attacker but has a useless ability (Minus which only helps with medicore pokes in Doubles).
I feel X has the superior Mega stones.
Now the version exclusives that aren't Mega Stones / Yveltel / Xerneas don't carry as much weight as they can easily caught again and traded but I'll include them as well.
Clawitzer in X is somewhat bulky with Mega Launcher that benefit its good coverage movepool but its dismal attack (has good physical attacks it can't take advantage of) and speed are kinda leave to only Assualt Vest and Choice sets. Dragalge in Y is bulkier with a unique typing of Dragon/Posion and has Adaptability (STAB becomes 2x instead of 1.5x), even with its worse speed its superior in usefulness. Y wins here.
Slurpuff and Aromatisse are both pretty meh fairy types.
Ok my rant is over, how about you?
