Although these two words have the same literal meaning, we tend to use them in different ways, especially when applying them to people. 'Incredible' usually has positive associations. If you hear someone say 'You are incredible!', in most situations we would understand this to mean 'You are amazing', in the sense of 'wonderful'. This is isn't always the case, but 'incredible' is used in a positive sense more often than not. 'You are unbelievable!' is most likely to be used in a negative sense, meaning, for example, 'I can't believe how stupid/thoughtless/selfish you are'.

I believe the two phrases to be very similar. The main difference would depend upon the context in which it is used. Please contact me for a class and I will be happy to discuss further. Dr. Priscilla/Professional Teacher