Terabytes of the same game, just in different colors.
Which is exactly why I stopped playing after Gen 2, which was the peak of the series IMO. If you want to see everything the series has to offer, just play Gold/Silver (or their remakes on the DS). No reason to waste time and money on any other Pokémon game.
Radical Red and Polished Crystal are peak, in my opinion. I can’t go back to the lack of special/physical split, it’s just awful.