Raichu and Jolteon have similarities and differences that overall make them a very even match for each other, but if I were to choose, I would choose Jolteon.
When looking at base stats, Raichu only edges Jolteon out in Attack, a stat that isn't entirely useful when it comes to Pokemon that learn mostly Electric-type moves. Not only does Jolteon have better Special Attack, it also has better Special Defense and Speed by quite a large margin.
However, Raichu is able to use a relatively larger movepool and its stats aren't too shabby either. Assuming we aren't using TMs, Pikachu can learn Thunderbolt (!!) at Level 26, as well as learning Slam at Level 20. Since Raichu has fairly powerful Attack and Special Attack, it would use these moves very well.
What I like about Jolteon is that you can get an Eevee and its respective Thunderstone in Celadon City, meaning you won't have to deal with the disappointing stats of Eevee. That being said, having a Pikachu early on in the game can prove to be very helpful. It's also worth noting that Eevee has other decent evolutions that you might want to use on your adventure, such as Vaporeon, Espeon and Umbreon.
Jolteon and Raichu are fairly even in comparison to each other. Choose whichever one appeals to you the most!
Hope I helped. :)
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