Wolf has pros and cons. His moves do a good amount of damage, but they do not have very good knockback, lessening your ability to KO an opponent with damage under 100%. In most cases, I have found that your opponent needs to be at around 150% before wolf can get a KO. Wolf is the least powerful of the three Star Fox characters, but he is also the heaviest, making it harder for an opponent to KO Wolf. Some would say that evens out the lack of knockback in Wolf's attacks. Overall, Wolf is a good character, and you have to respect him.
Pika, on the other hand, is nearly the opposite of Wolf. You may ask yourself, "How did they stuff all of this power in to such a little dude?" Pika is packed with power. He can KO opponents at around 70% with ease. His Skullbash and the Thunder shockwave have ecellent knockback. In order to even out Pika's power, he is lightwieght. He is one of the lightest characters in the game and can be KO'd fairly easy. But Pika has amazing recovery. He can recover from very far away horizontally using Skullbash, and he can even recover from underneath a stage using Quick Attack. Combine all his traits and Pikachu can be a pain in the neck to deal with.
In my opinion:
Pika > Wolf