There is no difference. These two sentences have exactly the same meaning.
If you're looking for a slight nuance, I'd say that the second puts a little more emphasis on the negative idea. For example, "I've had nothing to eat" is more emphatic than "I haven't had anything to eat". You might choose to stress the 'nothing' in this context, especially if you want to boast about how well your crash diet's going, or to complain or show how unhappy (or hungry) you are.
The difference is very subtle, however, and I really wouldn't worry about it. Just remember that English prefers to put the negative element of a statement as EARLY as possible in the sentence.