Ok here we go
Why Fox-Puff is a lot better than Marth-Puff, a thread:

As soon as Puff has a lead, she can camp Marth and force him to approach. She can't do that with Fox because of laser.

Marth has to rely on f-throw pivot f-smash to kill Puff early. Otherwise it's gonna be one long stock. Fox has upthrow-upair, easier gimps with shine, upsmash oos.

The problem with Marths only kill confrm aside from side-b uptilt (which I don't consider that good) coming off of a grab is that puff can crouch under Marths normal grab and rest him for whiffing. Puff can't crouch under Fox's grab.

That leads to another problem: If Puff rests Marth once, it's so hard to come back. With Fox having a reliable kill confirm at 50 and up-smash also killing around 60, a comeback with Fox is way easier. If I remember anything else, I will continue this thread later.