Depending on what you want to do. There are some differences like Bluetooth and bigger preview screen, but page printout is about the same.The visual cosmetics of each model are different.
PC World click here have both on a collect from store discount price. C7180 £299.99. C6180 £199.99. Their site also offers a good product description on both printers.
I have the very basic HP Photosmart C3180 all-in-one, and its giving problems at present, with overlap on website print-out. HP Support have advised on certain procedures, but these are not correcting the problem.