Modern Chaos Marines and WHFB Chaos Warriors are actually pretty damn distinct from one another. They used to look more similar than they do now, but their designs diverged greatly at some point.
Chaos Marines have an even chunkier, more flexible look to their armor, with the chest-pieces divided into different segments to reflect how they are supposed to move with the wearer. Their shoulders are more smooth and standardized, their legs and much chunkier and rounded, especially from the knees downward, and usually have less overlapping plates (because why wouldn't they? They're not literally medieval armor). Modern CSM also tend to have much more rounded helmets, and of course, they made it fashionable to have gigantic Khorne symbols stuck to the tops of their heads, which is the hallmark of 40K Khorne berserkers and only really was ever present in WHFB on a few select models. They also have a less culturally specific flavor - Chaos Warriors were definitely designed to have a viking element to their design, whereas Chaos Marines are supposed to be more modular in theme; they're designed with lots of flat surfaces and design space so GW can easily modify them to fit [x chapter theme], much like with loyalist Space Marines.
CSM and CWs are definitely variations on the same design, in that they're sci-fi and fantasy versions of a very similar idea. CSM are much, much more similar to the guy walking on foot in >>60011752
than they are to the average Chaos Warrior from Fantasy, though. At least that's how CSM have been for the last several editions.