I buy both just because. I have the money to buy it, so I'd like to experience both games when I could and not watch someone else play the game to see what I don't experience.
I usually buy both. When I do pick just one, I usually pick the most blue color. For example, Sapphire instead of Ruby, Diamond instead of Pearl, and so on. I did do Scarlet before Violet though, since I much preferred the past forms compared to the future forms.
I would usually just grab one based on the boxart. I think for X/Y I grabbed both and played them simultaneously. It was entirely too exhausting, though it did end up being one of my favorite regions.
I just grab the one that's more popular/loved. If I had to choose between both of the Lets Go games, I would go Pikachu, because lets be honest electric starters are a lot better than normal type starters. And if I recall correctly I don't think you can eeveelution your starter Eevee anyway.