Bloodstained is built for players that like the IGAvania games. The rpg-lite progression and weapon/skill systems have been honed over multiple games, so you are always getting stronger just by moving around. That has a certain appeal in itself- in the end you can brute force your way through the game as you become far more powerful than anything you find.
Hollow Knight is more like Metroid. Upgrades are largely found, and there's far more trade-off in offence/defence/mobility. It's exploration is far, far better, intending to make you feel lost when you first go (or even fall) into an area.
With Bloodstained, due to it's heritage I felt I knew the rough pattern of the castle before even getting past a quarter of it. There's a village, a castle, flooded caves beneath it, a clock tower, a garden, etc. With Hollow Knight, it's such a beautiful, fantastical work of imagination that I had no idea what to expect next. The city itself is beautiful, and the whole mournful atmosphere of a crumbling civilisation of bugs just tugs at the heartstrings.
Thing is, some people feel more comfortable with Bloodstained's familiar setup rather than feeling like they haven't made meaningful progression in a session. That's understandable. All in all, I'd say they appeal to different needs in terms of what a player wants out of a Metroidvania-type game (or 'exploratory platformer' or whatever). If you want the constant feeling of advancement and growing stronger regardless, a tried-and-tested way of keeping a player engaged which is why everything has rpg elements now, then Bloodstained is great. It's exactly what it's trying to be which is a comforting sequel to Order of Ecclesia etc, a romp through a haunted castle fighting demons and undead etc. I found it quite uneven in that, for all that the Castlevania action combat is addictive as hell (literally in this case), the character graphics on Switch weren't great, some skills felt way better than others, it crashed a couple of times and the village quests are such a tedious collection of fetch quests that they felt pointless, as did much of the crafting. However, it's been patched a couple of times since to balance some of those out.
On the other hand, if what you want is exploration in a fresh, fantastical setting, with less certainty that where you are going is the 'right' way forward, then the atmosphere of Hollow Knight will stay with you longer afterwards.
Personally I'd give HK the edge as it's polished in a way that Bloodstained isn't, but it really does depend on what style of game you want. I enjoyed them both but had largely forgotten BS a few months later, whereas HK stuck with me. Can't wait for Silksong.