Both are correct. I think the second is overly complicated, so the first is better.

It depends on context with other sentences. As a stand alone I would go with the 1st sentence. the second sentence I would also add a comma before which like : which.

These are both correct, they're just two different ways of expressing the same idea. The first one sounds more like normal, natural speech and the second is more formal, the kind you might see in an essay or a report.