The new Pokemon games have several Pokemon that can only be caught in one version of the game.
One of traditions of the Pokemon franchise is to release new main series games in pairs, each of which have their own set of "exclusive" Pokemon. This was originally set up to encourage players to trade with one another, and adds a little variety for players who pick up both games.
The version-exclusives are also one of the major determining factors as to which Pokemon game to buy, and Pokemon: Let's Go is no different. While there are only a handful of Pokemon that can't be obtained in one game or the other, those Pokemon might make all the difference as to which game you choose.
Of course, the biggest version-exclusive Pokemon in Pokemon: Let's Go is your Partner Pokemon. Although wild versions of Pikachu and Eevee are available in both versions of the Pokemon game, you're stuck with either Eevee or Pikachu as your Partner Pokemon depending on which game you buy. You won't be able to evolve your Partner Pokemon, but you can teach special moves that give it an edge even in later parts of the game.
In addition, the following Pokemon can be found in the wild only in Pokemon: Let's Go, Pikachu!:
And the following Pokemon can only be found in the wild in Pokemon: Let's Go, Eevee!:
We should note that all of the above Pokemon can be obtained either by trading with another player or by transferring Pokemon from Pokemon Go to Pokemon: Let's Go. Once transferred, these Pokemon can be captured in the Pokemon Go Park in Fuchsia City.
Pokemon: Let's Go, Pikachu and Pokemon: Let's Go, Eevee are available now.