word choice - Difference between kara and n desu?

Is there a difference (or nuance) or is it just two ways of saying the same thing?

For example, どうして、とぶか？　（１）　こわいんです。　（２）　こわいですから。

Obviously, those are very simple examples. But are there times where you couldn't use one in place of the other to explain "why" you did something (or you did something "because")?