I chose team Eevee I heard that you may not evolve the Eevee or Pikachu. I personally chose Eevee because Pikachu has been kinda outplayed by the anime.
Team Eevee without question. I was always indifferent to Pikachu, never liked or hated it. Eevee on the other hand is just so cute and even if it doesn't evolve, I like the move set Eevee gets and I could build a fun set of impractical moves where its harder to do that with Pikachu.
Team Pikachu for me. Pretty sure most of my friends will already be getting the Eevee version and I adore electric types, so I'll go for my original favorite Pika. =p whooo
I still haven't made up my mind if I'm getting one of these games, but if I do then it will definitely be Let's Go Pikachu. it's not even a question for me, really. Yellow was my first Pokémon game, which means the Pikachu version will be more nostalgic for me, plus I just don't really care for Eevee all that much. I mean, I like Eevee just fine but I feel like it's overrated.
I honestly really haven't decided yet. Eevee seems like the better choice but if you can't evolve your starter (which I expect to be the case), the differences are minimal. I'd have to discuss it with my brother, since he is getting the other version.