MeshCAM development slowed considerably when Carbide 3D was formed.
I haven't used it much in the last few years, so I don't know what features were added to 6 that 5 doesn't have. I wouldn't think there is much difference.
Imo, 7 is not really usable. But there have been a few releases, so it is being developed, just slowly. I actually prefere the interface in 6 to 7, so unless 7 added features, or fixed bugs, I'll probably keep using 6 for now.