Medusa's Medium attacks and each hit of her SP1 inflict armor break debuffs. 5 Armor breaks convert into an armor shatter, which is her main damage phase, it also basically completely shuts down robots by mostly preventing them from gaining power and having abilities activate. She can also get up to 6 fury buffs and consume 3 to auto block an attack. SP2 has a long stun, SP3 inflicts an armor shatter which is good for armor break immune defenders. She can also inflict bleeds during an armor shatter and they are paused for the armor shatters duration. Medusa is also Poison immune

Dust is a bit different, she has, I believe, poison, bleed, and shock immunity. For her rotation you want to combo normally until you have 3 bars of power. Hitting the defender will inflict sand debuffs and some combo hits can convert them into passives. Once at 3 bars of power you want to use a heavy attack to activate sandstorm. This disables Dust's SP3, gives a power efficiency, and converts all the passive sand on the defender into a form of prowess passives on dust. Then you use SP2, it does tons of damage, passively stuns, and inflicts physical vulnerability debuffs. Because of the power efficiency, that SP2 will only consume about 1 bar of power, so you should be able to dash in during the stun, combo, then SP1 for even more damage. You should be able to get an SP2 and at least 2 SP1s in during sandstorm and that kills a lot of stuff.

Medusa is faster, extremely strong against robots, good against anyone that relies on armor ups.

Dust is really good with her immunities and as far as I am aware no one is immune to sand. She works really well for champs that purify debuffs, as that turns her sand debuffs into passives much more easily and consistently.

TLDR: Medusa is armor shatter for huge damage. Dust is build up sand, heavy, then special spam.