Couple of biggies for the 9CS over the 7C...

first, 9C uses a module which is a big plus for frequency changes as well now you can go synthesized.

Second, 9C has better switch assignability....so you can customize your setup to how you like it

Third, more mixes and multipoint mixes. May not seem obvious at first or all that useful but down the road these mixes open up a lot of setup doors that otherwise you would not have.