The Cougar 500K has more in common with the 700K, not the 600K. The only differences I note between the 500K and the 700K is the 700K comes in four flavors of Cherry MX instead of membrane, the 700K has the aluminum framework, and has pass through connections for Mic and a headset, (I think that feature is useless as it is better to use the front panel jacks unless you are connecting them a bit more permanently.)
The Cougar 600K is built like the 700K but sacrifices the "G-keys" (macro keys) and has a full size spacebar. It also sacrifices the backlit keys, UIX, and mic and headphone connections.
I've read though while the 700K has mechanical switches, the mode (profile) keys and the multimedia keys are not mechanical.
I do have questions though, and it fits for ANY keyboard tested: What kind of quality are the keycaps? Will color or letters wear off or are they durable?