I feel both are just about right though I'd give bloodborne slightly on the harder edge because there is not much choices u have in taking down the bosses.

Dark souls 3 u can spec ur character to be warrior or spell caster or archer and that gives u more choices on how u wanna take down the boss.

Go for dark souls 3 if u want variety (tons of loot, weapons, armor that means a difference, shields, etc)

Go for bloodborne if u want a lite rpg version of devil may cry / ninja gaiden.