There are certain Pokemon you can only catch in the wild in one of the two versions.
Exclusive to FireRed:
Arbok
Delibird
Ekans
Electabuzz
Gloom
Golduck
Growlithe
Oddish
Psyduck
Qwilfish
Scyther
Seadra
Shellder
Skarmory
Weezing
Wooper
The Pokemon listed above also have another species of relatively the same type (e.g. Oddish in FireRed and Bellsprout in LeafGreen).
Exclusive to LeafGreen:
Bellsprout
Kingler
Magmar
Mantine
Marill
Misdreavus
Muk
Pinsir
Remoraid
Sandshrew
Sandslash
Slowbro
Slowpoke
Sneasel
Staryu
Vulpix
Weepinbell
The chances of encountering certain Pokemon may be different depending on which version you're using. A good example would be few male Nidorans in LeafGreen, yet plenty in FireRed.
Really, though, in all other aspects of the game, LeafGreen is the same as FireRed, for the most part.
Leafgreen can only trade between firered, ruby, sapphire, emerald, Pokemon colosseum, Pokemon xd gale of darkness, and migrate with diamond, pearl, platinum, heartgold and soulsilver it cannot trade with the original silver.
You can only mix records between Pokemon ruby, sapphire and emerald not leafgreen.
Red, Blue, Yellow, Gold, Silver, and Crystal can trade between themselves. Ruby, Sapphire, Emerald, Firered, and Leafgreen can trade between themselves. Games from list 1 cannot connect with list 2.
If you trade Pokemon between leafgreen and ruby ruby will have your leafgreen Pokemon and leafgreen will have the ruby Pokemon both in the party they will not be sent to the PC unless you personally walk to the PC and do so.
There is no emerald to trade between firered and emerald you need wireless adapters.
you can get Pokemon that are not in the other version
Firered and leafgreen both can catch different Pokemon but there pokedex is the same, here is some examples: magmar only in leafgreen not firered, oddish in firered not leafgreen, ekans in firered not leafgreen, bellsprout in leafgreen not firered, pinsir in leafgreen not firered.
It doesn't matter. The only difference between the two is that certain Pokemon battles with the same person will use different Pokemon.
You can trade between firered, another leafgreen, ruby, sapphire and emerald also Pokemon colosseum and Pokemon xd gale of darkness.
Latias and all other hoenn Pokemon are unable to be captured in both leafgreen and firered to get them you must trade between ruby, sapphire and emerald.
You would have to get it from firered or leafgreen the cool about firered and leafgreen is each can purchase the TM Dig at the Celadon City Dept Store then they can trade the TM to Emerald remember to trade between emerald and leafgreen or firered you must use wireless adapters.
Red, Blue, Yellow, Gold, Silver, and Crystal can trade between themselves. Ruby, Sapphire, Emerald, Firered, and Leafgreen can trade between themselves. Games from list 1 cannot connect with list 2.
Leafgreen can only trade between firered, ruby, sapphire, emerald, Pokemon colosseum, Pokemon xd gale of darkness, and migrate with diamond, pearl, platinum, heartgold and soulsilver it cannot trade with the original silver.
No you cannot trade from Game Boy Color And Game boy Advance.
lots of Pokemon can be only caught in firered and leafgreen like mewtwo and raikou, suicune, entei, articuno, zapdos, moltres. Plus the kanto starters bulbusaur, squirtle, charmander.
Crystal cannot trade to Leafgreen because for a couple reasons: One that the game is on a different console and is not compatible with the Gameboy Color, Two since the game had not came out when Crystal was released no program was created to trade between the newer games that would follow Crystal even if the newer games are remakes of the older ones such as Firered and Leafgreen.
one difference is that in fire red there are oddish but no bellsprout, and visa versa for leaf greenI'm not sure but I think the different Pokemons you can catchex.you can catch a bellsprout on leaf green while fire red cant(Or reverse)From:Reian Viloria