~ 중이야 is more along the lines of "I'm in the middle of ~" or "~ is in progress" rather than just simple progressive. In the case of 기대(하는) 중이다 and 기대하고 있다, the meanings are very similar, but -고 있다 can't always be replaced with ~ 중이다 or vice versa. Additionally, and this is just my interpretation, but 기대하고 있을게 to me implies that you will be expecting it when the time comes, whereas 기대(하는) 중이야 means that you are anxious about it right now.