Been thinking about the difference between these two types, and I've been able to clarify it to myself rather well - so I felt like writing it down so other people could tell me what they think.
Counter phobic sixes are often described as bring confused with 8s because of their confrontational stance. Both can seem belligerent and aggressive, so on the surface it can be hard to tell the difference. Of course as always with the enneagram, it lies within. The two types may act similarly, but will be doing it for two entirely different reasons.
8s are characterised by their lust for life and extreme groundedness within themselves - they plant their feet in the ground, speak the truth about themselves and others, and will never back down. "Here I am, don't like it? Deal with it." Their aggression comes from wanting to know the same qualities in others - where they stand, who they are, the absolute truth. Like the other body types, the issue is primarily relational - where do people stand, what are they doing and how do they carry themselves doing it?
With 6s, the issue is not truth - it's loyalty. 6s have trouble with standing on their own two feet, because like the other head types they are ruled by fear. It's like the ground isn't stable enough for them to remain truly centred with themselves, and so they become reliant on others to set boundaries, establish the playing field and assuage their doubt. In this sense, the six is really looking for where the boundaries are - how far can I push this until someone notices? The same can be said of the other extreme as is common of sixes), that they want people to play by the established rules and so berateabd attack others for not honouring them. "Where would we be if no one was keeping check?"
This has been very hastily typed at work, so apologies if it doesn't do a good job clarifying anything. Anyway, let me know what you think - I would appreciate it.