Well, Pikachu with a Light Ball has doubled attack stats, making it a better attacker. In Alola though, Alolan Raichu has just 5 less base special attack than Light Ball Pikachu. I’ll do a comparison of their base stats.
Pikachu w/ Light Ball: 35 HP, 110 Attack, 40 Defense, 50 Special Attack, 50 Special Defense, 90 Speed.
Kantonian Raichu: 60 HP, 90 Attack, 55 Defense, 90 Special Attack, 80 Special Defense, 110 Speed.
Alolan Raichu: 60 HP, 85 Attack, 50 Defense, 95 Special Attack, 85 Special Defense, 110 Speed.
So, as you can see, Raichu has more bulk and speed, I mean, it’s still going to be one-shotted by anything physical, but still (that’s an exaggeration, obviously SOME physical moves aren’t going to knock it out), they have more bulk and speed. I think Alolan Raichu is pretty superior to Light Ball Pikachu, having nearly the same Special Attack and Alolan Raichu having better stats all around (except for Defense, which they are tied in, and Attack, but using physical attacks on Alolan Raichu is very rare anyway).
So, in conclusion, evolve it in Alola, and if you’re not in Alola, Light Ball Pikachu is better. Also, Alolan Raichu’s shiny is amazing.
Hope this helps! :)