Thanks for all the advice! I plan on picking up Pokémon Green as I understand the Japanese versions of Gen 1 games are super different. Thanks again!
In essence, how it worked was this.
*The games were originally released as a set of two, Red and Green, which differed based on what Pokemon were available.
*A third version was then released which contained various bugfixes, plus redrawn graphics and an entirely different set of available Pokemon. The Pokedex entries were all rewritten, and the Unknown Dungeon was given a new layout.
So here's how that all worked out in regards to how the games were localized as "Red and Blue":
Japanese Red: Pokemon availability set A, graphics A, Pokedex entries A, Unknown Dungeon design A
Japanese Green: Pokemon availability set B, graphics A, Pokedex entries A, Unknown Dungeon design A
Japanese Blue: Pokemon availability set C, graphics B, Pokedex entries B, Unknown Dungeon design B
Western Red: Pokemon availability set A, graphics B, Pokedex entries B, Unknown Dungeon design B
Western Blue: Pokemon availability set B, graphics B, Pokedex entries B, Unknown Dungeon design B
Since Fire Red and Leaf Green were primarily based on Red and Green, these games were the first time the west got exposed to that original set of Pokedex entries and that original Unknown Dungeon design.
So if you want to experience a game you've already played with different graphics and a slightly different map at one point (which you've already seen in FRLG), get Red or Green. if you want to experience a game you've already played with a different selection of Pokemon available to you, get Blue.
Or just get both, because they're both going to be new to you in different ways.