Pichu's is better. Pikachu has more range and deals 4% more than pichu (pichu will deal 21%, pika does 25%), yes, and it can't be sdi'd out of, but look at the difference in knockback. Pichu's sends them like 2 or 3 times farther than pika's.
People can SDI out of pichu's fsmash, but that will rarely happen at low percent, which is alright because Pichu's kills are all gimps at low percent anyways. As for self damage, fsmash will give you like 2 damage... thats not significant, and on average, you will take 10%-20%(20% being you are spamming electric moves anyways) per stock total, thats not that big of a deal imo. the majority of your electric attacks only deal 1% to yourself.
The main way that Pichu uses fsmash is when preventing somebody from grabbing the ledge (in which case SDI'ing won't help them anyways) or out of a pawd, both are hard to escape.
The only advantage of pika's fsmash is the range and the fact that it can't be SDI'd out of, either way Pichu's is still better, you just have to use it smarter, its not something you can just throw out there.