Pichu Vs Pikachu - Pikachu is more powerful, but he's also bigger, slower, falls VERY SLOWLY, very vulnerable. Pichu is faster, jumps higher, falls quicker, rolls better, good for tricking opponents. It's kinda like Gdorf VS Cfalcon except a little less... extreme.
And all of you who said that these two moves are the worst:
Kirbys Down B
Shieks Over B
Obviously aren't thinking situation-wise or maybe you're just noobs. If you move the control pad in circles after you throw the chain with sheik he'll swing it around for up to 10 seconds. This is perfect for edge guarding, cave guarding, wall guarding, team battles, stalling, and a couple of other things.
Kirby's down B is useful when your opponent's are grouped up or trapped. Since there's no way to stop it from coming, it has virtually no lag time, and it's not bad for power either, it can be very helpful. I beat event 51 on my 8th try with it when I first got this game. It also makes a brilliant recovery move - you get sent flying and you use the move to get back down from a Star KO. Handy when your opponents have weapons, or are trying to juggle since A attacks don't effect it. The only way to stop it from hitting you is to roll to the side and grab.