I tried out some stuff, nothing being official or anything, just some quick tries because I felt like it.
My conclusions:
Luigi's up-B only makes the jiggly-sound when used on the ground, otherwise it just does minimal damage, rest can be used in mid-air and still kill, or at least do a lot of damage.
Ganon died at like 140% when using up-B right in front of him on the moving platform on the spear pillar stage and at around 150% on FD, either I'm doing something seriously wrong, or that really isn't what I call "stronger than rest".
It also killed jiggly at around 70%, and from what I heard, jiggly dies at 52% from a rest.
No DI involved in any of this.
But, yeah, no way are you going to punish a luigi when he hit you, and no way are you going to kill him when he missed, so it's way more practical, and rest is just far too situational and often suicidal now. It's a shame, I know.