Here are their base stats in the form HP/Atk/Def/SpA/SpD/Spd:
Pikachu (normal): 35/55/30/50/40/90 = 300 total
Pikachu (Light Ball): 35/110/30/100/40/90 = 405 total
Raichu: 60/90/55/90/80/100 = 475 total
Credit to Trachy for this graph. We can see that Raichu has higher HP, Defense, Special Defense and Speed. However Pikachu has significantly more Attack and Special Attack. It depends on your team.
TL:DR: Evolve for more Speed and bulk, however keep it for more damage.