I've played both so...
Bloodborne is a Souls game basically, which doesn't have as you've never played them. You basically have EVERYTHING at the start. You won't get new combos/skills/abilities/weapons much at all throughout the game. You basically pick a weapon and level it and your stats that correspond to it like leveling strength for a strength weapon or skill (basically dex) for a skill weapon. You will occasionally come across and get say a magic ability but if you don't level arcane, you won't be able to use it anyways. The Souls games are weird in the sense you actually level and get worse as at the start, you can really use any weapon effectively with base stats against the opening level enemies. Then, as you level, you level your weapon and weapon scaling stat (like strength) to stay the same (kill enemies in the same number of hits basically) while giving up your options of using other types of weapons (since you didn't level those weapons or those stats). You level to stay on par with enemies and give up other combat options, you don't really get better. And, there's no respec option in these games either, makes no sense. The combat is very solid; however, you will be using pretty much the same strategy vs just about every enemy. The Souls games are only hard if you can't play cautious or patient as pretty much every standard enemy is really easy to kill if you play it safe, meaning trying to only fight one enemy at a time and not trying to do cool shit in combat as you have to survive from lamp post to lamp post without dying to make progress. Any environmental traps are pretty easy to avoid by paying attention (I played through Dark Souls' trap filled dungeon without getting caught in one trap for example). It's not a hard game, it's more of playing in the proper mental state than acquiring badass combat skills. The only challenges the game really throws at you are the boss fights, which really aren't too hard (there's always a few challenging ones in there). The reason I have played Dark Souls and Bloodborne to completion is the level design and atmosphere, I just love exploring the levels in these games. If I played just for combat, I would've quit after 5 hours or so because it really doesn't change. Bloodborne is probably the hardest Souls game just because it doesn't let you play as cheap as the other Souls games like you can't hide behind a shield or be cheap with magic and the game does play at a faster pace. Bloodborne definitely has decent length but it's really not much of a time sink like other RPGs. Lastly, the story, the little there is, is of very little importance.
Firstly, I haven't finished Witcher 3 but I've really poured quite a bit of time into it so I can't really comment on how well the story is as I haven't finished it. Also, it is the 1st Witcher I've played. I will say the Witcher 3 starts really really slow. You start out as Geralt, a witcher is his prime, with just base skills for some reason. Thus, it takes awhile for the combat to get interesting as it wasn't until like level 10 that I started digging the combat. The action-based combat is a lot better than a Skyrim or Fallout. You have to "slot" every skill you get so at the start of the game, you only have one slot so you can only have one skill active thus it takes some leveling to open slots to use a few skills. The game does allow you to be rather cheap in combat as you can spam the shield power to basically lose no health when hit or spam the mind power to stun enemies over and over again. Plus, the early quests are really basic and uninteresting for the most part. There's a quest to find a frying pan, I realize the early quests are to teach you the mechanics but you could make the quest more interesting. After X amount of time played, I really did start digging the game a lot from the quests to the characters to the combat. There's lots of dialogue in Witcher 3. The boss fights, the few there have been (I'm at least at the halfway point), were very disappointing as the way to defeat the boss is extremely simple and the boss usually has a way to get health back, which only prolongs the boring fight. I say the boss fights are worse than Deus Ex HR boss fights. With that all said, I do feel Witcher 3 is a very solid open world RPG.
I really don't know what to recommend as they are very different games, but I do feel Witcher 3 is the better experience after a horribly slow start.