To me, those are the two most interesting types to contrast. I see them getting confused often so I went about delineating and exploring what distinguishes them. Disclaimer: I find it distracting to narrow it down to 8 versus cp6, or both SX variants. That can happen later, but it's important to see through the essence of the types first. And again remember, it's about motivation when talking enneagram, the instincts just show in which realm this motivation unfolds. Like 6 SO wants to be secure within a community or cause while 6 SX wants to be secure with something that connects to them like a significant other. With that in mind, here's the guide for you. Please comment what you think and want to add/suggest/correct, with IRL stories if you like.
SHADOW
One of the most prominent features of an Eight is that they become controlling and vengeful when unhealthy. They stop focussing on building their own strength and begin to take strength from others. They see external attempts to blanket them everywhere. They will boss up, asserting their will to create a feeling of being in charge, leaving another as disempowered as they dreaded they themselves would be. Their defense (aka attack) mode makes them spiral into fearing outward oppression more and more, their surrender is impossible. The Eight will become a megalomaniac shoving away everything that's between them and their throne. They drift from controlling themselves and defending others to controlling others and defending themselves. Their true innocent and vital nature becomes dictatorial.
A Six is in complete distrust when they are unhealthy. Everything in the world is a menace to them. Their suspicion and vigilance in order to feel secure develop into a paranoia aimed at conservation of what they can hold on to (hence Sixes are called Traditionalist sometimes). It has to be set and established for them, they don't destroy like an Eight would. Paranoia shields them from others, further making them worry about being forsaken. Their ability to be loyal and a troubleshooting companion is wasted as they are now withdrawing their faith out of insecurity. And so others withdraw, and the Six becomes even more suspicious and less devoted. And if they're counterphobic, they'll destroy what makes them insecure, causing more harm than good on both sides. The counterphobic Six has polarizing attitudes around caution. They would be the person beating up an Eight bully, both ending up hospitalized.
DECISION PROCESS & REACTIONS
The Eight makes decisions from the gut. That happens quickly and impulsively with confidence. There is less place for being wary since the eight goes the pragmatic route toward the strength they desire. They like to implement it all the way they want it directly. Their drive is to be self-reliant and potent, so there's no room for anxiety games, trust issues, or thinking what could go wrong since that downsizes them. They simply cannot allow themselves to be afraid.
The Six makes decisions from the head. They ponder, often getting into a spiral of worry. So it takes more time, especially for the phobic type. They can self-defeat. The counterphobic Six is willing to decide to `act against´ faster but again it comes from a calculated mindset. It's cerebral and in fright of different scenarios that they thought up. The Six won't be a visionary leading their fate, they navigate life to evade certain fates, especially their worst-case-scenario that's always present.
Eight is confident in their ability to handle things, they confront it all as it comes. They even provoke it! "Try me!" is the Eight's challenging phrase. Eights would rather be the one taking the first step, act > react. Their leader courage is inbuilt. They are the forward-charging improviser of the gut triad, with the Nine as the accommodator who tags along, and the One who micromanages. There's nothing fearful about their disposition. That would be weak anyway, which is unacceptable. To fear is to submit to danger already - and submission is the Eight's absolute kryptonite. They instinctively feel that they MUST project larger-than-life power (confer Ice Cube as a good example), otherwise they portray the opposite, subjugation. They'll step on you before you step on them.
Six prepares, observes nervously. React > act, their courage is lacking, the phobic Six in particular may end up as a coward. They will only provoke to test someone's standpoint out of feeling uneasy and quizzical. "Is it really like that?" is the Six phrase. They are way more future-oriented because that is where a potential threat rests - or could rest. A six chases a castle of glass more than often, and they designed it themselves since their experience tells them to distrust, to question. Nothing is ever safe enough for a Six. Reacting to anything, you will see their skepticism. It'll show in the body language, too. Observe Ryan Gosling who's counterphobic. He fidgets around, the trembles, he looks into every direction. You spot a Six by how they double-check, all the time.
RELATIONS
Eight would never choose to really chain themselves to anybody. If they have the opportunity, that is. Eights suffer under their lack of freedom in a way, especially with a seven-wing. But they already know about this lack of opportunity. The world, for the Eight, is unjust, unbalanced. As mentioned above, it's all about eating or being eaten, it's black-and-white here, and they just toughen up to deal with it as good as they can. They will relate to another only as the one with more power. Lust is powerful and takes instead of giving (ensuring dominance), which can motivate Eights into a union.
The Six needs a guide. Something or someone they can confide in. An orientation. People, a partner, certain property (this is where the instincts start). A sign, just anything. Even just an idea, especially when they wing at Five. Six is helpless otherwise. They are clingy when in their phobic mode, have no problem with being the less powerful one at all, as long as they are safe. The cp Six will lash out quick, but they still doubt and look for a higher guidance. Six actually makes a good match to an Eight who's the leader.
(DIS)INTEGRATION
The Eight can borrow from the Five's intellectual approach when under stress. They observe, even when it's not necessary. They suddenly want to outsmart what makes them feel weak. They hoard secrets for unfulfilled vengeance when they are unable to overcome adversary. Although they'd do better not closing themselves off from this enemy. The Eight integrating Two is big-hearted because the realize how they can love another, and how they can receive love which seems foreign to them in their natural strong/weak tug-of-war state. Then, they are ready and open for affection and can reconcile things without being subdued. Even the angriest Eight has the potential to become a generous people-pleaser who is adored.
The Six's desire to be secure through others can become a self-validation problem. Look at me! This cry of Three promises support from others. It's easier to say that than making all the efforts of being loyal oneself to earn loyalty in return. Disintegration always gives us a short route to deal with stress upon the core fear. In the case of the Six, wanting another person's approval can be a quick fix to get them the people to rely on. When that security is naturally given and the core fear at ease, the Six can become accepting and unity-seeking like a Nine, who is finally ready to be peaceful and not anxious.