Its blended equally from what i know. So 33/33/33. WPI WPC Casein.
Speaking strictly for 1kg of WPC vs the blend, it's $27 vs $29 respectively. For an extra $2 you get a third of it as casein, plus the benefit of a third being WPI and the added lactase.
I always prefer some casein, and normally throw a scoop in a shake anyway, so for me the blend is a no brainer.