As is often the case, Let's Go comes in two editions: Pikachu and Eevee. Both versions play exactly the same; they are the same game. So you don't need to buy both of them, unless you really want to. However, there are some differences between the two--some minor, some less so.
Your Partner Pokémon
The first and perhaps biggest difference is your partner Pokémon, who always accompanies you during your journey. In Let's Go Pikachu, your partner is Eevee... no, it's Pikachu of course! Meanwhile Eevee is your partner in Let's Go Eevee.
Your partner is basically your starter Pokémon--your very first. But there's a twist this time. Firstly, your partner will always be with you, although you can remove them from your party. Secondly, your partner cannot evolve, but they have enhanced stats and a variety of exclusive moves to make them stand out.
The main reason the Pokémon games come in pairs (besides making more money) is to encourage trading! For the most part, you will encounter pretty much the same Pokémon in both Let's Go Pikachu and Let's Go Eevee. But there are a few Pokémon that only appear in a specific version.
Therefore, if you're planning to catch 'em all, you'll have to trade with somebody who has the version you don't. That could mean somebody in real life. Or, thanks to modern technology, you can also trade with other players online. See, it's not so hard!
|Let's Go Pikachu||Let's Go Eevee|
|Sandshrew, Sandslash||Vulpix, Ninetales|
|Oddish, Gloom, Vileplume||Bellsprout, Weepinbell, Vileplume|
|Mankey, Primeape||Ekans, Arbok|
|Grimer, Muk||Koffing, Weezing|