Here's a breakdown:
FireRed:
* Starter: Charmander
* Exclusive Pokemon: Charizard, Venusaur, Blastoise (all available through trading)
* Gym Leader: Blaine (Fire-type)
* Elite Four Member: Lorelei (Ice-type)
LeafGreen:
* Starter: Bulbasaur
* Exclusive Pokemon: Venusaur, Charizard, Blastoise (all available through trading)
* Gym Leader: Brock (Rock-type)
* Elite Four Member: Bruno (Fighting-type)
Similarities:
* Both are faithful remakes of the original Red and Green.
* Both have the same storyline, characters, and gameplay.
* Both feature improved graphics and sound.
Which to Choose:
* Charmander fans: Go for FireRed
* Bulbasaur fans: Go for LeafGreen
* Want a challenging early game: Choose LeafGreen (Brock's Rock-type Pokemon are harder to deal with early on).
* Don't care about starters: Choose whichever version you find more appealing.
Ultimately, the "better" version depends on your personal preferences. If you can't decide, you might want to consider playing both!