Coke or sprite? Scrambled or sunny side up? Paragon or renegade? The world is full of difficult existential questions, and while you're on your own/SOL with most of them, we here at Kotaku would like to offer a helping hand in choosing between Pokemon X & Pokemon Y.
Some context, before we talk specifics: traditionally, the main Pokemon games release in two versions. Each version has creatures that the other version doesn't have, and this is partially done to encourage people to play Pokemon with their friends, and partially because, let's be real, Nintendo/Game Freak probably make more money that way.
The number of version-specific Pokemon has varied from one gen to the next, and honestly? In this latest iteration, there aren't too many version-specific Pokemon. Even if there were, the ease in which you can trade Pokemon with other people online means you don't have to worry too much about missing out on specific Pokemon.
Still, there are some differences between Pokemon X and Pokemon Y. I won't list all of the differences because that'd be spoiling the game, but let's compare the major Pokemon differences you should consider before purchasing, shall we? Most of these concern version-specific Mega Evolutions, which are temporary power-ups that some Pokemon can undergo in battle.
The differences: Charizard X changes from a fire and flying type Pokemon to a fire and dragon type Pokemon. Charizard Y, meanwhile, sees its special attack stat increase significantly, and gains the ability called "drought." Drought increases the power of fire-type moves.
The differences: Mega Mewtwo Y increases its special attack significantly and it gains the "insomnia" ability—as it suggests, this ability prevents Mewtwo from falling asleep. Mega Mewtwo X, meanwhile, becomes psychic/fighting type, and its attack stat increases. In addition, it also gains the "steadfast" ability, which increases its speed whenever it flinches in battle.
And now, the legendaries...
The differences: Xerneas is a deer-like fairy Pokemon with the "fairy aura" ability—which increases the damage of fairy-type attacks for all Pokemon in the battle. Yveltal, meanwhile, is a dark/flying type Pokemon with the "dark aura" ability—which increases the damage dealt by dark type attacks for all Pokemon on the field.
The last question you've gotta ask yourself before purchasing is: which letter do you like better, X or Y? Yes, I know that sounds silly, but trust me, it matters! People will ask you what version you got; it's like picking a sports team. That's part of what makes it all so fun, even if there aren't too many differences between the versions. UPDATE: based purely on anecdotal evidence, if you want to be a cool kid, you should pick Pokemon Y...most people seem to be getting Pokemon X.
With all of this in mind, you should be able to choose between the versions easily. So then! Now that we're here...which version will you be getting? Personally, I think Pokemon X has most of the cooler Pokemon, but hey. That's just like, my opinion, man.
PS...if you really don't mind being spoiled, here's the full list of all the version-specific Pokemon.