No offense to your question, but this is pretty much arguing about how many angels can dance on the head of a pin. You probably won't find a neat, side-by-side answer for this.
Between Bru and Pietra I prefer Bru. Off the top of my head, I figured it was because of her butt size. But that’s not necessarily the end of it, cause I actually prefer Mika Lafuente’s type over Bru’s, get what I mean?
Maybe we can't even explain our own taste. Sexual stuff is almost entirely hardwired into our primal brains.