I find these types of comparisons a little wearisome to be honest, but one thing to bear in mind is the number 6 doesn't mean the same thing in every side. Partey's role is very different to Casemiro's and the burdon on the former is far higher.
One more thing, I think Partey's passing ability is massively underrated. Genuinely in disbelief that it gets criticism from some. The pace, timing and accuracy of his passing is superb as is his decision making. It's one of the key reasons why teams struggle against us so much, because he switches play rapidly or disguises his pass. Take a closer look, it never slows the play down. It sets the tone for our fast paced football. It's actually one of the reasons I don't rate Rodri as much as others, because as good as he is technically and as secure as his passing is, everything is a little slower compared to Fernandinho.