If I had to guess the difference between the Sith and the Knights:
The Sith were individually selfish and only cared about self-survival. Their own title, "Dark Lord of the Sith", indicated they were the rulers of something (the "Sith").
When one hears about a group of knights, they imagine a group of warriors loyal to a cause, working together to serve and protect it. These are "Knights of" something, not "Dark Lords of" something. They aren't the rulers.
So, perhaps the Knights are concerned about the overall rule of the Dark Side, believing the power it holds is so great, that it's the right way of the galaxy. The Sith didn't see that far and only cared about individual domination.
Perhaps "Ren" is even a title for the Dark Side itself, who knows? That'd be a stretch, though.
And then there's Kylo, obsessed with the Sith/Darth Vader. Everything we've heard about him so far is how unbalanced he is, how driven by emotion he is. If the Knights are dedicated to the Dark Side completely and totally as a whole, as I mentioned above, then that would make Kylo the odd-one-out, looking at himself and comparing himself to a great Dark Sider of the past. Thus, that would give us a unique villain to focus on and follow on a journey deeper than blind devotion to the Dark Side.
I hope that makes sense! haha