Those are not sentences, they are just parts of sentences (called clauses). They mean the same thing. For example, here are two sentences that mean the same thing: - I was wearing a helmet, which is why I survived. - I was wearing a helmet, and I survived because of it.

They do mean the same thing but their applications (uses) are different. You could say something like "I fell on top of feathers, which is why I survived." Though this does sound a bit awkward, to my best knowledge, it's not wrong. I would avoid using this phrasing but it's definitely not wrong. You could also say something like "I have to give thanks for the feathers that broke my fall, because of them I survived." This is more common to hear and use, in my experience.