Well both characters are equally as fucked Yuki, however i believe in the +R Remake Slayer got hit a tiny bit harder with the nerf bat then millia. From what i've seen millia is an extremely fast rushdown character who has average damage output but is looking to end each combo in a hard knock down. This is so that she can set up her hard slash disc, which keeps the opponent in blockstun allowing you to go high or low, do the same combo and repeat until they either find a way out or there dead. However because of her bad health it is very important to be a moving target with the character otherwise you will die very quickly especially as if she doesnt have a dp of sorts (i havent seen any millia players use one anyway)
Slayer is just a close range god, he's got amazing close range pokes that go into hard knock down or launcher (depending on how much meter you have). Once you have 25% tension then the party can start with crouch hard slash (launcher) into force break big bang upper into combo, which when done right can easily remove 40-50% health. Plus on hard knockdown with his dash being a teleport he can very easily set himself up for a high low mixup allowing him to go back into hard knock down or big bang upper combo. However he suffers from a bad neutral game and can easily get zoned out, and although his pressure is good because all of his combos and strings are links there are holes in which the character can backdash and escape his pressure, which puts slayer at a disadvantage immediately.
However seeing your character is Chie both characters can suit you quite well, however Millia would suit you more imo simply because Millia has the same mixup tool as Chie (that being a move which keeps them in blockstun for days, allowing you to do whatever you want with them). Hope this helped