Its forward smash kills earlier than Pikachu's. If you want to play competitive, go with Pikachu since Pichu is just not worth the struggle.
- Smaller hit boxes make Pichu kind of hard to hurt in comparison to Pikachu. Not that much of a difference, but it's still there.
- Stronger forward smash. Pichu has a way stronger move than its evolved counterpart, and it can kill at medium-high percents when fully charged, and it benefits with rage.
- Pichu is literally the lightest character in the game so far, and the fact that it damages itself with half of its moveset doesn't do it any favors. Pikachu himself already has a bad time playing defensively, but he's a more effective glass cannon than Pichu is and will likely ever be.
Hope I helped.