We loved Kakadu! We have heard people compare it to Litchfield and had mixed points of over as to which one is better, and we have heard people say you cant compare the two. We found them to be completely different, I don't think I could say I liked one more than the other. Kakadu had a feel to it that is hard to describe, a feeling of expansion and connection that I didn't get a Litchfield (maybe because we were having so much fun in the water falls). This may have been due to the superb ranger talks and the wide uncrowded spaces, or maybe because of the extra distance and effort to get there. I dont think it actually matters as we enjoyed them both equally for different reasons.
In a nutshell, for us I think Litchfield was the fun park and Kakadu the real outback.