@molkom The differences entirely come down to some changes in which Pokemon are available, and the starters are the same in both versions. The question of which to go with pretty much entirely boils down to which set of exclusives you prefer –
FireRed – Ekans, Oddish, Psyduck, Growlithe, Shellder, Scyther, Electabuzz, Wooper, Murkrow, Qwilfish, Delibird, Skarmory
LeafGreen – Sandshrew, Vulpix, Bellsprout, Slowpoke, Staryu, Magmar, Pinsir, Marill, Misdreavus, Sneasel, Remoraid, Mantine