hmm, somebody correct me if i'm wrong, but as far as i can tell.
peach's does more damage...duh XD
priority...i'd also give to peach just cuz it seems to be more disjointed than pikachu's
knockback: peach; i've never KO'd anybody with pika's dsmash. ever.
i also think peach's dsmash starts up slightly faster too, but i'm not too sure.
so overall, i'd say peach's downsmash is better; they both have very similar uses except pika's might set up for an aerial or usmash if they have bad DI while peach's will ruin their stock if they have bad DI, haha.
but while we're on the topic of pika's dsmash...it's still useful:
-like i mentioned earlier, pika's dsmash may set up for some sort of follow up if you're lucky.
-it's probably one of your fastest moves in terms of start up: it can be a "get off me" move at times (but rising uair works better in most cases), or if you don't think you have a move that is quick enough, you might opt for the dsmash.
-it can be pretty good for gaining control of a match since it can get through some characters' aerials
-it can be used like peach's dsmash for edgeguarding (mostly spacies, falcon, and ganon). if people aren't expecting it or don't know about it, this will KO them.
-overall i'd say it's a pretty good defensive smash but not so much an offensive smash(who cares?? usmash ftw! =p) unless you get lucky.
