I would lean towards Ditha. (Though I haven't tried Louise)
My highest ELO score is from using Ditha in mono Pussycats during a week where Roots/Gheist were unpopular. Her potential for a 14 life gap when all your other cards damage reduce is really good. I often sink all 12 pill into her.
"Louise costs more for a reason"
This isn't always a good indicator.
Xu52 costs 3 times more than Rolph and isn't anywhere near as good a card.
Selma is half the cost of dalhia, but they're pretty much balanced.
I think both louise and ditha are equally great. All depends if you prefer attack power or gap potential.