I loved both, but Bloodborne just recently replaced ZOOT as my GOAT. For about 75 hours in I though BOTW was the greatest game I ever played, but it really lacks in story, the world at times can feel a bit empty, and the dungeons/bosses are not on par with most Zelda games. Don't get me wrong, BOTW is in my all time Top 10.
Bloodborne just blew me away with its epic bosses, enemy variety, aesthetic, and fascinating/deep lore and story.
I've said this for a while, but if Nintendo can make a BOTW sequel with more compelling bosses and dungeons and a more focused story, then Zelda could once again be king of the mountain for me. BOTW's open world was amazing, but why does the world need to be instantly unlocked; I missed the feeling of getting new items in dungeons and opening the world more gradually.
Why you can't quantify this stuff, I did this breakdown for fun:
Story/Lore - BB (this is not really close, BB's lore is fascinating and unbelievably deep; I could watch videos about it for hours)
World/Setting - Draw - In terms of aesthetic, atmosphere it's BB; in terms of interactivity with the world and variety BOTW
Gameplay - I lean BB; they both obviously control well, BOTW a little more refined and with more gameplay variety; but BOTW can never match BB's intensity and the feeling of accomplishment for besting a boss.
Enemies - lol, BB I mean c'mon
Bosses - lol, BB I mean c'mon
Art Style - BB, although I really like BOTW's art style as perfect for a Zelda; Yarhnam though is just amazingly well executed vision of Victorian and Lovecraftian horror.
Replayability - I'll say Draw - I think BB is more replayable from start to finish and for trying different builds and the DLC and Dungeons add a ton of content; But I put 140+ hours into BOTW although I've never had the urge to play it again or the DLC. The amount of content in BOTW is just amazing though
Music/Sound - I'd give sound to BB and music to BOTW. BB certainly had better voice acting.
Innovation - BOTW - that open world man, its amazing. They schooled everyone on their first attempt at open world.
fwiw, I just platinumed BB; played it after BOTW, GoW, and RDR2. Have not played Witcher 3, but will get to it.