Dedenne has superior stats to Pikachu:
Dedenne: 67 / 58 / 57 / 81 / 67 / 101
Pikachu: 35 / 55 / 40 / 50 / 40 / 90
As you can, Dedenne outclasses Pikachu is every stat. Dedenne's typing is better, being able to fare against Dragons.
However, with Pikachu's signature item, Light Ball, its Attack and Special Attack double. This is the only way Pikachu is superior. Also, Pikachu does has a slightly better movepool, having Grass Knot, ExtremeSpeed, and more moves from events.
So I would go with Dedenne unless you have Light Ball.