If you're keeping a Pikachu, and it has a choice of Volt Tackle, I'm going to assume it has a Light Ball. If so, still go with thunderbolt. While Thunder and Volt Tackle are both stronger than thunderbolt, thunder has 70 accuracy, while Volt Tackle has 90 accuracy and 1/3 recoil. Pikachu are incredibly frail, so you don't want to throw your precious HP out the window just to use Volt Tackle.
Remember, Pikachu isn't the high-tier Pokemon he is in the anime. Pikachu, without a Light Ball, are incredibly delicate and can die to any attack. Even with a Light Ball, they are still frail, but are hole-ripping powerful offensive forces.