When used as a surprise expression, the two phrases are about the same. For example, I don't see much difference between these sentences: - 야, 차가 참 좋으네요! - 야, 차가 참 좋은데요! (Both means "Your / Someone's car is gorgeous!"). The -네 ending is mostly used in this kind of expressions, while -는데 is used for other cases more often, like giving contradictory situation or opinion.