You should use X if you want a physical attacker because Raichu has many good OP moves that are physical, like Volt Tackle, Thunder Punch, Play Rough, Dig, Wild Charge, Facade, Iron Tail, etc. and it has the balanced stats.
You should use Y if you want a special attacker because Raichu has a staggering 160 Sp. attack and Nasty Plot, making moves like Thunder, Thunderbolt, Surf, Swift, Hyper Beam, Focus Blast, etc. more powerful.
Pros
Raichu has more physical moves than special, meaning that X's 135 attack is not wasted.
Raichu knows Nasty Plot and Agility, along with many powerful special moves (above), so Y is recommended for sweeping - It also has 160 sp. attack and 130 speed, already buffed out for sweeping.
Cons
Raichu X is slower by 20 speed points than Y, making X vulnerable to Nasty Plot - Agility sweeping...
Raichu Y has a terrible defense stat of 58 - Beware Mega Gyarados with Earthquake or Charizard X with Dig or Flare Blitz!!!
