Apples and Oranges ... like why even compare the two?
I prefer neither over the other because I can't compare them.
Hollow Knight is the best Metroidvania in the last year for me, not much comes close, expansive map, great bosses, good movement, good combat. A bit on the easy side for what I like in platformers, the only thing interesting platforming-wise was White Palace for me and that is way too short.
Dead Cells is my favorite Roguelike in some time, I like the difficulty modes, great combat, super fluid movement, good item selection, good balance. I dislike some mechanics like the capped boss damage and I think the game needs more endgame variations, at least one more boss on the same level as timekeeper and some optional biomes on the high castle level, maybe an actual optional endboss after HotK.
IF you were to force me on a decision what game I would take over the other I would pick Dead Cells because it has more potential to keep me interested over a long time, I played through Hollow Knight and outside of trying to speedrun I don't see me returning. Dead Cells will keep me entertained longer just by difficulty alone.