Difference between KMC Z6 and Z7 for 6 speed cassete?
Z6: from http://kmcchain.us/chain/z6-br-gy/ 

Z7: from http://kmcchain.us/chain/z7-2/ 

I think the only real difference is colour: Z6 is "silver" Z7 is "grey/brown"  This is important to some people.

Note, Z6 was renamed from Z33, and Z7 was formerly called Z50.

I think your links show the wrong image for Z6 - the KMC site shows plain chain without slots, so CRC have a bad image.